In early

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

time, an extensive

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc ...

system called the Sierran Arc began to develop along the western margin of the

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

n continent. In

Southern California Southern California (commonly shortened to SoCal) is a geographic and Cultural area, cultural List of regions of California, region that generally comprises the southern portion of the U.S. state of California. Its densely populated coastal reg ...

, this volcanic arc would develop throughout the

Mesozoic Era The Mesozoic Era is the era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms s ...

to become the geologic regions known as the

Sierra Nevada Batholith The Sierra Nevada Batholith is a large batholith that is approximately 400 miles long and 60-80 miles wide which forms the core of the Sierra Nevada mountain range in California, exposed at the surface as granite. The batholith is composed of ...

, the Peninsular Ranges Batholith, (in the

Peninsular Ranges The Peninsular Ranges (also called the Lower California province) are a group of mountain ranges that stretch from Southern California to the southern tip of the Baja California peninsula; they are part of the North American Pacific Coast Range ...

), and other plutonic and volcanic centers throughout the greater

Mojave Desert The Mojave Desert (; ; ) is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. Named for the Indigenous peoples of the Americas, indigenous Mohave people, it is located pr ...

region. Douglas M. Morton, Fred K. Miller, Peninsular Ranges Batholith, Baja and Southern California, Geological Society of America, 2014
/ref>

Geology

These massive belts of plutonic (intrusive) and volcanic (extrusive) regional belts and isolated centers developed as plate convergence and

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

took place farther west along the western

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental marg ...

. These igneous provinces shed vast quantities of sediment both eastward into the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

and westward into Pacific margin basin. At the same time, older sedimentary materials and rocks were subjected to regional

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

throughout much of Baja and

. In the region today, granitic rocks of Mesozoic age dominate the bedrock exposed in the Peninsular Ranges, western Transverse Range, the southern Sierra Nevada, and the greater Mojave region. Although igneous activity in the Southern California region was ongoing throughout the Mesozoic Era, the peak of the plutonism in the southern Sierra Nevada region was in the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

, about 100 to 80 million years ago. In addition, thick sequences (accumulations) of Mesozoic-age sedimentary rocks, mostly marine

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

and Cretaceous age, are locally preserved along the western side of the Peninsular Ranges and throughout the western

Transverse Ranges The Transverse Ranges are a group of mountain ranges of Southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa Ba ...

in parts of the

Santa Monica Mountains The Santa Monica Mountains are a coastal mountain range in Southern California, next to the Pacific Ocean. It is part of the Transverse Ranges. The Santa Monica Mountains National Recreation Area encompasses this mountain range. Because of its p ...

and mountainous

Los Padres National Forest Los Padres National Forest is a United States national forest in Southern California, southern and central California. Administered by the United States Forest Service, Los Padres includes most of the mountainous land along the California coast ...

region north of

Santa Barbara, California Santa Barbara (, meaning ) is a coastal city in Santa Barbara County, California, of which it is also the county seat. Situated on a south-facing section of coastline, the longest such section on the West Coast of the United States excepting A ...

. A thick sequence of terrestrial sedimentary rocks are also preserved in the McCoy Mountains region near

Blythe, California Blythe () is a city in eastern Riverside County, California, United States. It is in the Palo Verde Valley of the Lower Colorado River Valley region, an agricultural area and part of the Colorado Desert along the Colorado River, approximatel ...

. Except for the sedimentary rocks mentioned above, most of the Mesozoic-age rocks in preserved in Southern California display intermediate to high grades of metamorphism, typical of material that may have been buried to mid-crustal depths (probably in a range of 10 to 20 kilometers). Although the region was no doubt extensively covered with terrestrial and marine sedimentary deposits that interfingered volcanic deposits from the volcanic centers, most of this material was stripped away by erosion following regional uplift that continued into the following

Cenozoic Era The Cenozoic Era ( ; ) is Earth's current geological Era (geology), era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, Insect, insects, birds and flowering plant, angiosperms (floweri ...

References

{{reflist Geologic provinces of California Sierra Nevada (United States) Transverse Ranges Peninsular Ranges Santa Monica Mountains Mesozoic volcanism Volcanic arcs

Geology

See also

References