8.2: Rifting in the Basin and Range

Continental Rifting and Associated Features

Continental rifting (in fact all rifting) occurs when a tectonic plate is subjected to tensional stress, leading to extension and thinning. This process is an important part of the Wilson Cycle, a concept that describes the origin and breakup of supercontinents, such as Pangea and Rodinia, throughout geologic time. This cycle is directly correlated to the development of plate boundaries (see Plate Tectonics).

Figure \(\PageIndex{1}\) is a simplified summary of the stages involved in the Wilson Cycle. In this image, the first stage of the cycle is continental break up (part A of Figure \(\PageIndex{1}\), which begins with a series of interconnected fault zones and fracture zones that link regions impacted by mantle plumes. As the cycle progresses (in part B), rifting has progressed to the point that two regions on either side of the rift are now separated by a narrow sea. Fracture zones called "failed rifts" extend away from the sea, and volcanic centers related to the original plume locations persist. With progressive changes in plate motiion, extension and tension is converted to convergence and compression (part C). This phase results in the buildup of fold-belt mountains in the region that was once the narrow sea. With continued geologic change, the cycle of rifting repeats itself anew.

Figure \(\PageIndex{1}\): A summary of the the Wilson cycle of rifting and collision. (A) Continental rifting initiates. As rifting progressed and sea floor spreading initiated, the passive continental margin developed (B). Sediments of the Paleozoic were deposited in this setting. (C) Continents re-converge some time later, forming a fold-belt mountain chain. (D) Separation repeats. "Wilson Cycle" by Steven Earle, is licensed under CC BY.

When the continental crust experiences rifting, distinctive geological relationships result (Figure \(\PageIndex{2}\)). As the crust is stretched, normal faults thin and extend the crust. In the upper levels of the crust, this thinning produces sub-parallel belts of uplifted ranges and down-dropped valleys filled with sedimentary rocks. Earthquakes occur within the seismic zones of the range bounding faults. At the same time, the lower crust stretches and thins in a ductile manner (see Earthquake Mechanics). As rifting progresses, rocks in the footwalls of normal faults are uplifted and rotated, exposing geologic features at the surface that might otherwise not be seen. As basins in the hanging walls of normal faults drop down, sediment eroded from the uplifted footwalls fills them, preserving a record of uplift and erosion, as well as extension. The thinning allows mantle rocks to move closer to the surface, resulting in decompression melting and volcanism throughout the region, and volcanic features are often formed along the normal faults.

Figure \(\PageIndex{2}\): Major features of continental rift zones. Continental rift zones reveal long mountain ranges separated by deep valleys (basins) that are partially filled with sedimentary and volcanic material. Earthquakes (white stars) occur when the fault lines separating the basins and ranges suddenly let go. "Continental Rift Block Diagram" by NPS is in the public domain. Access a detailed description.

There is definitely a lot to interest geologists, ranging from outcrops of older rocks in uplifted footwall ranges, to sedimentary units in the down-dropped basins, as well as rift-controlled volcanism and even earthquakes!

Query \(\PageIndex{1}\)

Rifting in the Basin and Range

The continental rifting that is occurring in the Basin and Range Province is somewhat different from the style of rifting discussed in the context of the Wilson Cycle or the development of divergent plate boundaries (see Plate Tectonics). Rather than developing a single valley, rifting in the Basin and Range is widely distributed over many kilometers and has not yet organized into a single oceanic spreading center.

In the Basin and Range region, it is likely that interactions between two tectonic plates led to the formation of this broad rift zone. Geological work has shown that during the Late Cenozoic, changing plate motions led to the development of a transform boundary to the west of this region which eventually evolved into our modern San Andreas Fault System (see A Brief Geologic History of California). As this happened, much of western North America began to feel the effects of this changing motion as tension, resulting in widespread faulting and crustal thinning which produced the distinctive topographic basins and ranges of this region (Figure \(\PageIndex{2}\)). Although the age of the onset of extension is variable across the entire Basin and Range, within California, it initiated at roughly 17 Ma, with most activity occurring between 11 - 7 Ma.

Video \(\PageIndex{1}\): Tectonic Evolution of the Western United States

The video animation that follows presents a model for how this region has extended as the plate boundary to the west developed. There is no narration for this video, but a detailed text description can be found below.

Detailed Video Description

1. Present (last frame of movie). At Present the Pacific Plate fills most of the northeast Pacific Ocean basin with only the small Juan de Fuca and Cocos plates remaining from the previous configuration. The Pacific plate is moving northwest past North America. It has captured some slivers of the continental edge and is carrying them northwestward toward Alaska. Thus, the present Pacific-North America plate boundary lies within the continent, along the San Andreas fault system. It is connected to other plate boundaries at three-plate triple junctions, the so-called Mendocino and Rivera triple junctions; the locations and motions of these triple junctions help determine the on-shore geology in each time and place.
2. Early Cenozoic, 38 million years ago (first frame of movie). In the early Cenozoic, 40 Million Years Ago, other oceanic plates lay between the Pacific and North American plates. They were spreading away from the Pacific plate and subducting beneath the rim of the continent.
3. East Pacific Rise Migration. Coming forward in time, the eastern edge of the Pacific plate moved steadily northeastward as new seafloor was accreted by sea floor spreading. Eventually, the spreading center itself reached the subduction zone and the intervening plate was destroyed. The Pacific plate began to break off pieces of North America and carry them along the coast, creating the San Andreas-Gulf of California plate boundary inside the continent. Triple Junction Evolution. The Mendocino triple junction migrated steadily up the coast, attached to the Mendocino fracture zone on the Pacific plate. The Rivera triple junction hovered near the southern California borderland then, about 12 million years ago, when sea-floor spreading and subduction ceased off Baja California, it jumped to its modern position at the mouth of the Gulf of California.
4. San Andreas System Evolution. The past evolution of the San Andreas fault system occurred in two stage process. First the Salinian and borderland pieces of the continent were gradually transferred to the Pacific plate, later Baja California was transferred.
5. A third stage has begun: the Sierra Nevada/Great Valley block is in the early stages of being transferred and carried away. Pacific Plate Motion. It continually moved off to the northwest.
6. Basin and Range Expansion. In the early Cenozoic, western North America was much narrower. During the late Cenozoic it expanded, forming the Basin and Range province. This occurred during the time that the Pacific plate was pulling the rim of the continent away to the northwest, perhaps causing the expansion or, more likely, just making room for it. (It was already elevated, hot and weak, ready to fall apart given the chance.)

Based on studies of faults across the entire Basin and Range Province (not just that in California), east-west extension is estimated to have roughly doubled the distance across this region at the latitude of Las Vegas, NV. Geodetic data and geologic studies of recent earthquakes indicate that extension continues today.

Video \(\PageIndex{2}\): Basin and Range-Measuring Extension

This video illustrates the way GPS has been used to measure ongoing extension in this region. The video contains no audio; a cartoon animation of two GPS stations move apart as heat rises from the mantle toward the surface. At the same time as it is pushed upward by the heat below it, the tectonic plate stretches and thins and the crust fractures as normal faults form in response to the tension.

References

1. Affolter, M., Bentley, C., Jaye, S., Kohrs, R., Layou, K., & Ricketts, B. (2020). Historical Geology. https://opengeology.org/historicalgeology/
2. Burchfiel, B. C., Cowan, D. S., & Davis, G. A. (1992). Tectonic overview of the Cordilleran orogen in the western United States. In The Cordilleran Orogen: Conterminous U.S. (Vol. G-3, pp. 407-479). Geological Society of America.
3. Earle, S. (2019). Physical Geology. BCCampus Open Education. https://opentextbc.ca/physicalgeology2ed/
4. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2017). An Introduction to Geology. Salt Lake Community College. https://slcc.pressbooks.pub/introgeology/
5. Nelson, C. A. (1981). Basin and Range Province. In The geotectonic development of California (Vol. 1, pp. 203-216). Prentice-Hall, Inc.
6. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved June 28, 2023, from https://pubs.usgs.gov/of/2004/1007/geologic.html