8.3: Rift-Related Faulting in Eastern California

Normal Fault Structures in the Basin and Range

Throughout the Basin and Range, extension related to rifting happened via normal faults of two styles: horst-and-grabens and half-grabens/tilt block faults. These features form in response to tension and are usually oriented perpendicular to the tension direction.

In horst-graben structures (Figure \(\PageIndex{1}\)), two normal faults face one another such that the region between them is the hanging wall for each. The formation of these is described in As crustal tension begins, tension cracks start to propagate downward into the brittle part of the crust. With continued tension, these cracks evolve into normal faults that dip steeply toward one another, sharing the same hanging-walls (the graben). As extension progresses, the grabens between them drop downward to create symmetric valleys and the footwalls of each (the horsts) move upward, creating topographically higher ranges. Graben valleys often contain broad shallow lakes as run-off from the uplifted ranges is trapped in them. As extension progresses, these zones of horst-graben structures are repeated, forming a broad region of down dropped valleys separated by uplifted ranges. As they age, fault scarps are progressively covered by erosional debris shed from the uplifted ranges.

Half-grabens are a one-sided version of a horst and graben where a series of parallel normal faults develop (Figure \(\PageIndex{2}\)). As extension progresses, these faults thin the crust and the hanging-walls drop downward to form asymmetric valleys separated from asymmetric ranges that are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. When these basins are down dropped, they fill with sediments that are shed off of the adjacent uplifted ranges.

Figure \(\PageIndex{1}\): Basin and Range topography formed by repeated horsts and grabens. Such repetition and relief is caused by ongoing extension and crustal thinning. "Basin and Range topography" by NPS/Trista L. Thornberry-Ehrlich is in the public domain.

Figure \(\PageIndex{2}\): The evolution of tilted fault blocks in response to crustal thinning. Here, normal faults tilt in the same direction and rotate hanging walls as the crust thins. Some research propose that the ductal lower crust flows upward to support these newly thinned basins. "Tilted Blocks" by Aturn4000, is licensed under CC BY-SA 3.0.

In either case, the normal faults that develop are brittle and generally tilted at a high angle relative to the surface of the Earth, dipping at approximately 30-60 degrees. There is some discussion about what happens to them at depth, however, as pressures and temperatures of the middle and lower crust are more likely to produce ductile deformation, rather than brittle. For this reason, many people consider that normal faults are “rooted” in ductile shear zones, and some have proposed that the lower crust flows upward in response to the thinning of the upper crust.

Video \(\PageIndex{1}\): Basin and Range Structures

This video explains the way the features in the Basin and Range are related to rifting processes. The video includes useful narrated animations that further illustrate the origin of the “basin and range” structures in this geomorphic province.

Box \(\PageIndex{1}\): Metamorphic Core Complexes

Rocks in the uplifted footwalls of Basin and Range normal faults give geologists a way to study geologic processes that occur at deeper levels in the crust and quantify the temperatures and pressures under which these processes occur. Exposures of rocks that were deformed and metamorphosed at deeper crustal levels provided an opportunity to advance our understanding of normal faulting! The two important and related features that were recognized in this region are the detachment fault and the metamorphic core complex.

When first studied in the Death Valley region, geologists noted that regions of uplifted metamorphic rocks seemed to have gently sloped, somewhat rounded surfaces; these features were called “turtlebacks” after their sloping, somewhat rounded appearances. As geologists working throughout the west began to compile their findings, they realized that these metamorphic “cores” were often found in the footwalls of gently dipping mylonitic fault zones; the term “metamorphic core complex” came into use to describe these features. Inset Figure \(\PageIndex{2}\) illustrates the stages in the formation of these structures using simple diagrams. Stages in the development of a metamorphic core complex start with crustal thinning that stretches the lower crust and creates faults in the upper crust. The ductile lower crust buoyantly balloons toward the surface along a developing detachment surface. As the detachment surface evolves and extension progresses, upward movement of the ductile lower crust progresses, bringing deep crustal rocks closer to the surface.

Inset Box Figure \(\PageIndex{2}\): Stages in the formation of a metamorphic core complex. "Core Complex" by ManuRoquette, is licensed under CC BY-SA 4.0

Mylonite is a sheared metamorphic rock. It forms by intense ductile deformation of rocks in & along fault zones deep in the crust, and is commonly associated with metamorphic core complexes. The features of the mylonite are used to infer the sense of motion on the detachments and can also indicate the temperature and pressure conditions of movement (Inset Figure \(\PageIndex{2}\)).

Inset Figure \(\PageIndex{2}\): A photograph of a mylonite from Virginia, USA. The photograph is a cross-section view of the layered mylonitic foliation on a wet, cut surfac). This rock is oriented such that the sense of shear is parallel to the top and bottom of the image. The top of the image is being sheared to the right and the bottom is sheared toward the left; alternating bands of light mineral material contain asymmetrically rotated clasts that stand out amid the darker intervening layers of mica. "Mylonite (Rockbridge County, Virginia, USA" by James St. John, is licensed under CC BY 2.0.

The gently dipping mylonitic shear zones (called detachment faults or low-angle normal faults) were very controversial when they were first described. Most normal faulting occurs at high angles, more than 30° from horizontal, typically around 60°. Detachment faults, on the other hand, were shallowly dipping structures. Scientists believed they either started as faults with steep dips and rotated to lower angles with extension, or they formed at initially low angles, which pushed the known limits of rock mechanics at the time. Geologists simply couldn’t understand how these low-angle structures could be active! Over the years, however, geologists realized that there are many of these structures throughout the west, and they are also found in other mountain ranges around the world (such as the Himalayan Mountains), as well as on the ocean floor.Notable examples of metamorphic core complexes with detachment faults are found in the Funeral Mountains and the Black Mountains near Death Valley, but also in regions of the Mojave Province that are impacted by Basin and Range extension (the Whipple Mountains).

Pull-Apart Basins and Oblique Normal Faults

Although east-west tension is the dominant tectonic stress in the Basin and Range region, it is not the only stress: the San Andreas transform boundary to the west generates shear stress as well. The combination of these forces creates a type of basin called a ”pull-apart basin”; Death Valley is a perfect example in which half-graben structures can be observed. Death Valley is situated between the Panamint Mountains to the west and the Black Mountains to the east; the Amargosa Range is to the north-northeast. Panamint Valley is to the west of the Panamint Range.

As shown in Figure \(\PageIndex{3}\), a right lateral strike-slip fault to the south of Death Valley transitions to a normal fault separates the Panamint Range from Panamint Valley on the west, with the Panamint Range in the footwall. To the northeast, a right lateral strike slip fault transitions to a normal fault to the south, separating Death Valley from the Black Mountains to the west. Together the system of normal faults and right lateral strike slip faults create an overall sense of tension that is oriented northwest-southeast, and act together to pull the ranges apart from one another, thereby downdropping the Death Valley and Panamint Valley grabens and uplifting the adjacent Panamint and Black Ranges as east-ward tilted range blocks.

Figure \(\PageIndex{3}\): Death Valley's pull-apart basin formed between the Panamint Mountains and the Black Mountains. The ranges are half-graben tilt blocks as described in the text. "Death Valley's pull-apart basin" by NPS/Christian Poutre & J. Jurado, is in the public domain.

In other areas of the Basin and Range Province, normal faults are actually oblique faults (Figure \(\PageIndex{4}\)), which move partly along dip directions (normal motion) and partly along strike directions (strike-slip motion). This combination is formed when faults are not completely perpendicular to the tension or shearing direction.

Figure \(\PageIndex{4}\): An oblique slip fault combines normal and shear (strike-slip) motion. In the block diagram, a fault plane cuts horizontal strata and tilts downward to the right. The offset is a result of normal motion and left lateral strike slip motion; the hanging wall on the right is dropped downward and also slides to the left relative to the footwall block "Oblique Slip Fault" by NAishik Rehman is licensed under CC BY-SA 4.0.

Ongoing Faulting and Seismicity in the Basin and Range

The Basin and Range Province in California remains seismically active at present. While most of the current relative plate motion between the North American and the Pacific plates is accommodated by the San Andreas fault, approximately 15-25% of the total motion is also taken up by the right-lateral oblique normal faults of California’s Basin and Range Province and others to the north in western Nevada. These zones of active faulting form a seismic belt called the “Walker Lane”, which may represent an incipient major transform fault zone which could replace the San Andreas as the plate boundary in the future!

Most of the faults within the Death Valley area have been active in the Holocene epoch and some are active at present. A good example of the seismic risk in the Basin and Range Province is the Ridgecrest region south of the Panamint Range and just north of the east-west trending, left-lateral Garlock fault (Figure \(\PageIndex{5}\)​​​​).

Throughout this region, normal faults and right lateral strike-slip faults tend to be oriented north-northwest and separate uplifted ranges in their footwalls (such as the Sierra Nevada Mountains, the Inyo Mountains, the Panamint Range and the Black Mountains) from down dropped basins (such as the Owens Valley, Panamint Valley and Death Valley). Fresh fault scarps and active seismicity throughout this region (which has been the site of many moderately sized historical earthquakes extending as far back as 1946) attest to ongoing tectonism. The most recent was a series of shallow strike-slip events south of the Panamint Range that culminated in an M_w 7.1 which produced 2.5 m of right lateral offset occurred in 2019.

Figure \(\PageIndex{5}\): Map of NNW-SSE trending normal and strike-slip faults in the Death Valley area that have been active during the Quaternary period. Faults separate the ranges of the Death Valley region from the adjacent valley floors. The Garlock fault trends east-west to the south of Death Valley. The epicenter of the Ridgecrest Earthquake is south of the Panamint range, east of the Panamint Valley fault and north of the Garlock fault. "Death Valley Area Faults" by Mikenorton is licensed under CC BY-SA 4.0.

Video \(\PageIndex{2}\): Basin and Range Earthquakes

This video demonstrates the way in which earthquake hazard is monitored in this region. The silent animation explains that the build up of stress along a normal fault leads to rupture, which creates earthquakes. Seismic waves travel away from the epicenters and reach monitoring stations.

An important historical example of the Walker Lane risk is the 1872 Owens Valley earthquake – also known as the Lone Pine earthquake, which struck on March 26, 1872 in the Owens Valley. The epicenter was near the town of Lone Pine. Its magnitude has been estimated at M_w 7.4 to 7.9, with a maximum Mercalli Intensity Scale rating of X (Extreme). This earthquake was one of the largest earthquakes to hit California in recorded history and was similar in size to the 1906 San Francisco earthquake. Twenty-seven people were killed and fifty-six were injured. The earthquake resulted from sudden vertical movement of 4.6 - 6.1 m (15 - 20 ft.) and right-lateral movement of 11 - 12 m (36-39 ft.) on the Lone Pine Fault and part of the Owens Valley Fault; the scarp can still be visited and studied today. The scarp for a segment of this fault is shown in Figure \(\PageIndex{6}\). The scarp is developed in poorly sorted alluvial fan deposits and is approximately 4.6 - 6.1 m (15 - 20 ft.) high.

Figure \(\PageIndex{6}\): The Lone Pine Fault Scarp cuts across the alluvium. The person is standing on the down-dropped hanging wall and the boulder covered slope behind is the upthrown footwall. "Lone Pine fault scarp 1200px" by Mav is licensed under CC BY-SA 1.0.

The Lone Pine earthquake of 1872 was so intense that naturalist John Muir famously wrote about experiencing shaking in Yosemite Valley over 180 km (115 miles) away (Muir, 1901):

"...one morning about two o'clock I was aroused by an earthquake; and though I had never before enjoyed a storm of this sort, the strange, wild thrilling motion and rumbling could not be mistaken, and I ran out of my cabin, both glad and frightened, shouting, "A noble earthquake!" feeling sure I was going to learn something. The shocks were so violent and varied, and succeeded one another so closely, one had to balance in walking as if on the deck of a ship among the waves...

Then, suddenly, out of the strange silence and strange motion there came a tremendous roar. ... and I saw ... falling ... thousands of ... great boulders ...pouring to the valley floor in a free curve luminous from friction, making a terribly sublime and beautiful spectacle-an arc of fire fifteen hundred feet span, as true in form and as steady as a rainbow, in the midst of the stupendous roaring rock-storm. It seemed to me that if all the thunder I ever heard were condensed into one roar it would not equal this rock roar at the birth of a mountain talus."

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