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 motion, extension and tension are 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 begins again (part D), resulting in re-separation.

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 rift valleys, or basins. Lava flows and other volcanic materials are deposited in these basins, along with river and lake sediments as they fill with sedimentary rocks and volcanic layers. 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 older geologic features at the surface fault escarpment 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 as magma reaching the surface erupts from volcanoes and fissures 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 is available.

 

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

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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