California Geologic History

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California Geologic History


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Why do Sierra Nevada look this way?


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Introduction

Californias geologic history is very complex, most of the statedid not exist as a coherent piece of the earths crust untilrelatively recently in geologic terms.

Rock formations that are now adjacent often have very differenthistories.

Some of the larger collisions were associated with majorepisodes of tectonic activityintrusive and extrusive volcanicactivity, folding and faulting, and mountain building.

The most recent period of mountain building is still going on, andpractically all of the current landforms and geographic featuresare very young in geologic termsonly a few million years old.


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Proterozoic and Paleozoic Era

(2,500-245 mya)

Rocks older than 600 million years are rare inCalifornia.

The oldest rocks, which are more than 1,000 millionyears old, are located in the eastern deserts and theeastern Transverse Ranges (San Bernardino and SanGabriel Mountains).

The distribution of rocks of these ages suggests that thewest coast of the North American Continent was well tothe east of all but the southern end of what is nowCalifornia.


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All of these very old formations have been extensivelymetamorphosed-difficult to determine the conditionsthat existed when they were originally formed.

Some of the oldest (around 1,800 million years old) arelocated in the mountains around Death Valley and aremuch like the rocks exposed in the inner gorge of theGrand Canyon.

Metamorphic rocks around 1,000 million years old in theSan Gabriel Mountains and the Orocopia Mountainseast of the Salton Sea.


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1,000 million years ago and 400 million years agoappears to have been quiet in western North America.

The coastline remained east of California, probably inUtah and Idaho. Very thick sections of marinesedimentary rocks from this period are exposed in themountains east of the Sierra Nevada. For the most part,these rocks are carbonates (limestone and dolomite),indicating shallow, warm ocean conditions with littleincoming terrestrial sedimentpossibly similar to theGulf of Mexico today.


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Antler and Sonoma Events

Around 400 million years ago the quiescent period inwestern North America came to an end.

Antler Orogeny-Development of an offshore island arc. Theevidence for this event is strongest farther east, but there areindications in the northern Sierra Nevada and Klamathregions. Possibly similar to Japan today.

Sonoma Orogeny-250 mya-This event is most evident in

central Nevada, which may have been the location of thecoast at the time. The orogeny seems to have marked thejoining of the island arc containing rocks now found in theSierra Nevada foothills and central Klamath Mountains withthe rest of the continent. This event appears to mark the first

time the coast moved west into most of California.


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Mesozoic Era (245-65 million

years ago)

Nevadan Orogeny 210 mya. Uplift of the

ancestrial Nevadan Mountains caused by the

formation of the main body of intrusive graniticrocks that now make up most of the Sierra

Nevada (subduction zone probably similar to the

current situation along the West Coast of SouthAmerica).


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Cretaceous

Forces that created the intrusive granitic rocks of

the Sierra Nevada and Southern California

continued episodically until about 90-100 millionyears ago. During the same period sedimentary

deposits were accumulating on the continental

shelf, slope, and in an offshore subduction zone.


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Cretaceous

By that time the subduction zone had shifted west toabout the current location of the Coast Ranges

Deposits in the subduction zone were mixed with rocksfrom the upper mantle beneath the crust producing theFranciscan Formation (extends from northern SantaBarbara to the northern California coast).

The oceanic crustal material in the Franciscan formationwas thrust up against the Sierran block by about 75million years ago, extending the boundary of the North

American plate further west.


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Cenozoic

This deposition continued without interruption

until around 40 million years ago. Over the next

10 million years the coast shifted back and forth,producing a patchwork of marine and non-

marine sedimentary rocks in the Coast Ranges

and western Transverse Ranges.


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Cenozoic

25-29 million years ago. The oceanic plate that hadbeen subducting beneath the western edge of North

America became completely overridden, starting in thesouth, and the North American and Pacific plates cameinto direct contact for the first time.

Tangential motion and expansion replaced convergentmotion as the North American plate began interactingwith the Pacific plate.

The San Andreas Fault system was formed.

In far northern California and the Pacific Northwest,

north of the Mendocino triple junction, convergentmotion has continued ri ht u to resent times.


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Cenozoic

Volcanic activity, related to extension and thinning of the crust,became widespread in the Sierra Nevada and Mojave regionsaround 20 mya.

Around 10-15 mya a series of deep marine basins formed alongthe coast between Orange County and the San Francisco region.The appearance of the basins may have signaled the passage ofwhat is now coastal southern and central California over thespreading zone.

The rocks formed in these basins (Monterey Formation) arecomposed mainly of material derived from marine organisms,rather than terrestrial sediments.


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Cenozoic

About 5 million years ago mountain-building activity rapidlyaccelerated, and finally most of the modern mountain rangeswere uplifted, including the Sierra Nevada and the large fault-

block ranges to the east, the Coast Ranges, the TransverseRanges, and the Peninsular Ranges. San Andreas Fault jumpsinland.

Subduction continued in the north forming the major volcanoes ofthe Cascades.

Pleistocene glaciation in the Sierra Nevada and, to a minorextent, in the San Bernardino Mountains; recent volcaniceruptions in the Mojave and Great Basin regions; and thewidespread volcanic activity that created the southern Cascadevolcanoes (Mt. Shasta and Mt. Lassen) and the lava flows of theModoc Plateau region.


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The big knot:

Killer

Earthquakes

In the future!


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SUMMARY

Accreted terranes

Nevadan orogeny

Farallon plate

San Andreas Fault

Present uplift


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California Geologic History

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