Natural Provinces of California
NEED TO LOOK AT MAP WITH PROVINCES AND GEOLOGY
The first step in learning about the geology of California is learning its geography. We will learn about California geology from looking at the natural geomorphic provinces.
There are eleven natural provinces, each characterized by a unique landscape. California has the most awesome geology and geography of the lower 48 states.
It is certainly the most varied by having both the highest peak and lowest valley. The landscape ranges from broad flat valleys (like what you see in Illinois and Kansas) to the jagged, glaciated Sierra Nevada. California has active volcanoes that speak of enormous catastrophic eruptions as well as gentle flowing lavas like those of Hawaii.
Let’s start from southern California and move north.
The Colorado Desert province is the on-land extension of what major geologic structure?
The Colorado Desert province is the Salton trough, which includes the Salton Sea and the Imperial Valley.
The Salton trough is dominated by the San Andreas Transform system that borders the northeastern part of the trough. The San Jacinto Fault zone borders the southwest.
The Salton Trough is 72 meters (236 ft.) below sea level.
Geologically, the Salton trough is a pull-apart basin.
Meaning that the San Diego and Baja California Peninsular Ranges are pulling apart from the rest of North America.
Basically, the Salton trough wants to be like the Mid-Ocean Ridge East Pacific rise which is a divergent, rift tectonic margin that comes up through the Gulf of California and onto land as the San Andreas Transform system.
Geographically, the Salton Trough is known for agriculture irrigated by water from the nearby Colorado River.
In 1901, the Colorado river bank was cut to divert ‘some’ water into the Imperial Valley.
In 1905, five large floods hit the Colorado corridor causing the entire Colorado River to be diverted into the Salton trough.
Basically, within a couple of weeks the Salton Sea was artificially/ accidentally made.
The current lake level is maintained by diverting water from the Colorado.
Most of the water (~1 million acre feet in 2001) comes as runoff from nearby farms. This needs to be cut down to 700,000 acre feet by Dec. 2002. The saved water will be transferred to cities in southern California.
Given our water woes in California, is it prudent to continue diverting water into the Salton Sea where it will just evaporate?
Rocks of the Peninsular Range are most like what other province?
Geologically, rocks of the Peninsular province is dominated by Mesozoic Granitic rocks like those of the Sierra Nevada.
Geographically, the ranges in this province include the San Jacinto, the Santa Ana, Santa Rosa, and Laguna mountains.
However, about 85% of the Peninsular province lies outside of California, continuing south into Baja California for nearly 1300 km.
What is the obvious, most unusual feature of the Transverse Ranges province?
The Transverse Ranges are a geological oddity in the North American Cordillera.
If you look at the general trend of nearly all of the ranges in western North America, they all trend N-S.
The unusual feature of the Transverse Ranges is that they trend E-W. Geographically, the dominating ranges in the Transverse province are the Santa-Ynez, Santa Monica, San Gabriel, and San Bernardino.
Geologically, the Transverse Ranges owe their oddity to the San Andreas Fault and the Big Bend (we will discuss this in more detail when we talk about the San Andreas Fault).
What major geologic structures define the NW and SW boundaries of the Mojave Desert?
Think of the Mojave Province as a wedge of Desert being shoved up against the Sierra Nevada by the San Andreas Fault.
Geologically, the Mojave is bounded by the Garlock Fault to the NW and the San Andreas to the SW.
Geographically, its landscape is somewhat similar to that of the Basin and Range province, but is more subdued.
BASIN AND RANGE
Name at least two famous valleys in the Basin and Range province of California?
Why is the Basin and Range so arid?
What two major factors may have played a role in the 1980 Mammoth Lakes earthquake swarm?
The Basin and Range province in California is the westernmost limit of a large natural province that extents from the Wasatch Range in Utah and all the way south into the Mexican States of Sonora and Chihuahua.
The province is a desert basically because of the Sierra Nevada rain shadow. Who can tell me what a RAIN SHADOW IS?
There are two areas in California (small area in the northeastern corner of the state and a much larger area bordering the Sierra Nevada and the Nevada border).
Geologically, the Basin and Range province is a zone of extension in the North American crust (sort of like what is happening in the Salton trough), and this extension begins with the Sierra Nevada Fault.
Geographically, the Basin and Range contains three famous valleys, the Owens, Panamint, and Death valleys (86 meters below sea level).
The major range in California’s Basin and Range is the White-Inyo Mountains (4341 m). [x 3.28 ft./meter = 14,242 ft.]
Also, the Long Valley-Mono Lake area of the Basin and Range near Mammoth Lakes has had very recent volcanic activity.
In 1980, a series of magnitude 6 earthquakes shook the resort area forcing the USGS to signal a volcanic warning for the area. The earthquakes were interpreted as 1) the result of magma moving closer to the surface and 2) seismisity along the Sierra Nevada Fault.
Geographically, the Sierra Nevada is by far the most dominant structure in California Geology.
It is an immense topographic barrier. It is over 640 km long and rises to elevations above 4,300 meters [14,104 ft.].
It basically runs from Lassen Peak in the north to the Garlock Fault in the south.
(Note: It is redundant to say Sierras or Sierra Nevada Mountains because Sierra is a plural form and is Spanish for Mountains.)
What is the general geologic structure of the Sierra Nevada?
Geologically, the general structure of the Sierra is that of a granitic batholith about 600 km long by 100 km wide. [724 mi. x 121 mi.]
The granitic rocks are the roots of a continental volcanic arc system that existed 70 to 166 million years ago.
Basically, what is happening right now in the Andes of South America occurred in California back then.
The Sierra Nevada are also characterized by Paleozoic or older metamorphic rock that speak of a much different tectonic setting for California.
What is the approximate thickness of sedimentary rocks beneath the central Great Valley, and what was the source of the sediments?
Geographically, the Great Valley may be considered the counterpart of the Sierra Nevada.
It is a narrow valley extending nearly 800 km with an average width of 65 km. It starts near Redding and continues to Bakersfield.
The Sacramento River in the north and the San Joaquin River in the south dominate the valley.
Much of the valley has an elevation close to sea level.
Geologically, the Great Valley contains an approximately 20,000 m thickness of sediments collectively known as the Great Valley sequence. There is additional 10,000 to 20,000 feet of Cenozoic sediment over this.
It is famous for petroleum and natural gas deposits.
You may ask, where did all of this dirt come from?
Just look the east.
When we discuss the Great Valley we will see that much of the material was derived from the Sierra Nevada as it was being uplifted and eroded. (In fact, the chemistry of minerals near the base of the Great Valley sequence can be related to the oldest volcanic rocks on the Sierran crest).
The California Coast Ranges are dominated by the San Andreas Transform system. This includes the Hayward, Calaveras, San Gregoria, San Andreas faults.
When was the largest historical earthquake in California, where did it occur, and what was its geological significance?
Be able to follow the general trace of the San Andreas Fault through California.
Where is the Carrizo Plain, and what geologic feature is it famous for (p.317)?
Where does the San Andreas Fault cross Highway 17?
Geographically, the Coast Ranges form a series of northwest trending ranges separated by parallel river valleys.
The province extends about 1000 km from the Klamath Mtns. in the north to the Transverse Ranges in the south.
Geologically, tectonics of the San Andreas Transform system dominate the province.
We will discuss the Salinian block (a piece of the southern Sierra Nevada transported by the San Andreas?), the Franciscan Formation and the California state rock, serpentinite, and the local coast ranges (Santa Cruz Mtns., Diablo Range, Gavilan Range, and Santa Lucia-Big Sur region).
The province is known for having the largest recorded historical earthquake, the 1906 San Francisco quake, which tuned early 20th century geologists onto the scale and magnitude of the San Andreas Transform system.
CASCADE AND MODOC PROVINCES
What is the general age and rock type found in the Cascade Range and the Modoc Plateau, and why are the landforms in these provinces so different?
What is the plate tectonic setting of the Cascade Range?
Young volcanic rocks and volcanoes dominate the Cascade and Modoc provinces. Geographically, the Cascade and Modoc provinces form the southern extensions of topographies that dominate the neighboring states to the north.
The Cascade province is dominated by its skyline of the volcanoes Shasta and Lassen.
The Modoc province is a plateau dominated by lava flows and smooth shield volcanoes.
Geologically, the province owes its landscape to the tectonics off the northern California coast.
The San Andreas Transform ends near Mendocino and gives way to a convergent-type margin where the Gorda and Juan de Fuca oceanic plates are being subducted beneath California.
The rocks of the Klamath province are most like those of what other province?
Geographically, the Klamath province lies in the northwest corner of the state.
The major ranges are the Trinity Alps, Marble Mtns., the Salmon Mtns., and the Siskiyou Mtns.
All of these ranges are about of equal elevation between 1500 and 2000 meters. Geologically, rocks of the Klamath province are most like those of the Sierra Nevada. However, the geology of the Klamath Mountains is more complex.
We will concentrate on the granitic rocks and compare them to the Sierra Nevada.