13.3: Physical Geology - Mountain Ranges

Other than the Pacific Coast, the Transverse Ranges and the Peninsular Ranges are the two most important physical landscapes in the region. Both ranges have their characteristics, from the trend of the mountains, to the different climates within each range.

The Transverse Ranges

As mentioned in unit 7, The Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The name Transverse Ranges is due to their east–west orientation, making them transverse to the general northwest–southeast orientation of most of California's coastal mountains.

The ranges extend from west of Point Conception eastward approximately 500 kilometers into the Mojave and Colorado Desert. The geology and topography of the ranges express three distinct segments that have contrasting elevations, rock types, and vegetation. The western segment extends to the San Gabriel Mountains and San Gabriel fault. The central segment includes mountains that range eastward to the San Andreas fault. The eastern segment extends from the San Andreas fault eastward to the Colorado Desert. The central and eastern segments (near the San Andreas fault) have the highest elevations.

Most of the ranges lie in the California chaparral and woodlands ecoregion. Lower elevations are dominated by chaparral and scrubland, while higher elevations support large conifer forests. Most of the ranges in the system are fault blocks and were uplifted by tectonic movements late in the Cenozoic Era. West of Tejon Pass, the primary rock types are varied, with a mix of sedimentary, volcanic, and metamorphic rocks, while regions east of the pass are dominated by plutonic granitic and metasedimentary rocks.

Tectonics

The Transverse Ranges result from a complex of tectonic forces and faulting stemming from the interaction of the Pacific Plate and the North American Plate along the dextral (right slip) San Andreas Fault system. Their orientation along an east–west axis as opposed to the general northwest–southeast trend of most California ranges results from a pronounced left step in the San Andreas Fault that occurred in the Pliocene (4 million years ago) when southern reaches of the fault moved east to open the Gulf of California. The crust within the Pacific Plate south of the ranges cannot easily make the left turn westward as the entire plate moves northwestward, forcing pieces of the crust to compress and lift.

Figure 13.11: The San Andreas Fault & General Direction of Plate Movement. Image by Wikipedia is in the public domain.

Prior to this shift of the fault to create the left bend, northwest–southeast trending rock belts in all the Transverse Ranges began to rotate clockwise in the right shear of Pacific Plate – North American Plate motion. This tectonic rotation began in Early Miocene Time and continues today. The total rotation is about 90° in the Western Transverse Ranges and less (about 40°) in the eastern ranges. Catalina Island shows the most rotation: almost 120°.

A mechanism proposed for the rotation event is capture of the subducting Monterey plate by the outboard Pacific plate. Because the Monterey plate was then beneath southern California, the capture resulted in pulling of the overlying crust out and northward from the rest of California.

The rocks of the Transverse Ranges exhibit extreme differences in geologic age and composition, varying from sedimentary rocks in the western Santa Ynez and volcanic rocks in the Santa Monica Mountains to primarily granitic and metamorphic rock in the central and eastern segments, including the San Gabriel and San Bernardino Mountains.

The oldest basement (deepest) rocks are of Proterozoic age and are found in the San Gabriel Mountains and the San Bernardino Mountains. The Jurassic-Cretaceous Franciscan Assemblage is found in the western section of the ranges and is the presumed basement in this segment. Exposed plutonic rocks from the Mesozoic, mostly granites, can be found on Mount Piños and generally in regions east of Tejon Pass. The youngest rocks are Cenozoic sedimentary and volcanic rocks that can be found throughout the ranges.

The western segment is distinctive for the large thickness of Cretaceous and Cenozoic sedimentary rocks, estimated to be up to 10 kilometers. The thickest deposits of these are in the Santa Barbara Channel and Ventura basin. These are mostly marine in origin with a marked change to red beds of river systems of Oligocene age in the western and central segments.

Limestones and dolomites of the marine Miocene Monterey Formation are found in the Santa Ynez Mountains and in the Coast Ranges to the north.

Faults

The distinctive feature of the Transverse Ranges besides their anomalous orientation is that they are bounded by east-west trending faults. Most are left slip, strike slip faults. In the western and central segments many of the faults are thrust faults. Faults in the Coast Ranges and Peninsular Ranges trend northwest-southeast and butt into the east-west trending faults of the Transverse Ranges. Because all these faults are considered active and seismic, but they do to cut each other, the only geometry that satisfies that observation is if the east-west faults and Transverse Ranges are rotating clockwise with respect to the faults outside that province.

Figure 13.12: Map Faults in Southern California, as well as the Transverse Range. Image by Wikipedia is licensed under CC BY-SA 4.0.

Between the western segment and the Peninsular Range to the south is the complex Malibu Coast—Santa Monica—Hollywood fault, which exists as the border between these two mostly geologically unitary provinces. These faults are part of the same thrust fault system south of the northern Channel Islands of San Miguel, Santa Rosa, and Anacapa Islands. North of the western segment the bounding faults are left slip Santa Ynez and Pine Mountain Faults. The San Gabriel Fault and San Andreas bound the central segment. The eastern segment is bounded by the Pinto Mountain Fault in the north and the Salton Creek Fault in the south.

Field Trip: Avenue S. & The San Andreas Fault

Let’s head on a field trip in search for San Andreas Fault! Either Scan the QR code or visit this link to learn more about the San Andreas Fault, and to visit the pressure ridge created by it in Palmdale, California. (Video length: 3 min).

The Peninsular Ranges

The Peninsular Ranges are a group of mountain ranges in the Pacific Coast Ranges, which stretch over 900 miles from southern California in the United States to the southern tip of Mexico's Baja California peninsula. They are part of the North American Coast Ranges that run along the Pacific coast from Alaska to Mexico. Elevations range from 500 ft to 11,500 ft (150 m to 3,500 m) and vegetation in these ranges varies from coastal sage scrub to chaparral, and from oak woodland to conifer forest.

The Peninsular Ranges of southern California include the Santa Ana Mountains, San Jacinto Mountains, and the Laguna Mountains. The Peninsular Ranges of Baja California include the Sierra Juarez, Sierra San Pedro Martir, Sierra de la Giganta, and Sierra de la Laguna. These ranges run from north to south.

The Santa Ana Mountains are the largest natural landscape along the coast of southern California. These mountains peak at about 5,689 feet, on Santiago Peak. This range starts in the north, in the Corona area heading southeast of the Puente Hills region.

Geology

Rocks in the ranges are dominated by Mesozoic granitic rocks, derived from the same massive batholith which forms the core of the Sierra Nevada Mountains in California. They are part of a geologic province known as the Salinian Block which broke off the North American Plate as the San Andreas Fault and Gulf of California came into being.

Between this set of ranges and the Transverse Ranges is the complex Malibu Coast—Santa Monica—Hollywood fault, which exists as the border between these two mostly geologically unitary provinces.

A point of interest is the Pala District, which is just north of Escondido in the San Diego-Carlsbad area. After United States annexation of California following its victory in the Mexican–American War, Pala became known for its mineral resources, including gold and tourmaline. More than 20 gem mines were established in 1890s, which still produce tourmaline today, with the pink variety as the regional specialty.

China's Dowager Empress Cixi of Qing Dynasty highly prized the pink tourmaline mined in Pala. Under her influence, China's demand for this gem created a boom in the California tourmaline industry after 1902, particularly at the Himalaya mine. Demand fell off about 1911, declining after the Empress died in 1908.

Pala was the site where morganite beryl was first discovered. In 1902 it was found to have the first commercially significant deposit of kunzite, named after George F. Kunz, the godfather of gemology.

Peaks of the Peninsular Ranges

The San Jacinto Mountains are in the desert areas in the north and east side of southern California. They peak at about 10,833 feet. They run from the San Bernardino Mountains southeast to the Santa Rosa Mountains. This mountain range is the northernmost part of the Peninsular Range.

The Santa Rosa Mountains are located at the southern end of the San Jacinto Mountains, where they connect to it. The range extends for approximately 30 miles (48 km) through Riverside, San Diego, and Imperial counties, along the western side of the Coachella Valley, where they bound the Anza-Borrego portion of the Colorado Desert. The highest peak in the range is Toro Peak (8,717 feet).

The Laguna Mountains are in the eastern part of San Diego County. They range northwest to southeast for approximately 20 miles and peak at Cuyapaipe Mountain (6,378 feet). These mountains extend northwest about 35 mi (56 km) from the Mexican border at the Sierra de Juárez. The Sonora Desert lies to the east and the Santa Rosa Mountains are to the northwest.

Figure 13.13: Inspiration Peak in the Laguna Mountains- Looking Towards Anza Borrego Desert. Image by Wikipedia is licensed under CC BY-SA 3.0.