California Geologic History

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California Geologic History

• Part I Pre-San Andreas Fault System

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Acknowledgement

• Slightly modified from a presentation by
• Mark W. University of Colorado at Boulder
• http//snobear.colorado.edu/Markw/Mountains
  /03/week10.html
• With some slides from
• Chris Hill Grossmont College
• http//www.grossmont.edu/chrishill/californi
  a_geology_powerpoint_pr.htm

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Think-Pair-Share

• What do you know about our states geology,
  present and past?

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http//www.consrv.ca.gov/cgs/geotour/PublishingIma
ges/GeolProvinces2.jpg
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Geologic map of California

• What story this map tell?
• Activity Tour of the map
• How are different rock types grouped
• Where are the oldest rocks found?
• Where are the youngest rocks?
• Can you find the Franciscan Complex?

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Overview Its Complicated

• Californias geologic history is very complex
• Most of the state did not exist as a coherent
  piece of Earths crust until relatively
  recently in geologic terms.
• Rock formations that are now adjacent often have
  very different histories.
• Terrane A rock formation or assemblage of rock
  formations that share a common geologic history.
  A geologic terrane is distinguished from
  neighboring terranes by its different history
  (USGS)
• Multiple episode of orogeny or mountain-building
• The most recent period of mountain building is
  still going on, and practically all of the
  current landforms and geographic features are
  very young in geologic termsonly a few million
  years old.

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Proterozoic and Paleozoic Eras (2,500-245 mya)

• Rocks older than 600 million years are rare in
  California.
• The oldest rocks, which are more than 1,000
  million years old, are located in the eastern
  deserts and the eastern Transverse Ranges (San
  Bernardino and San Gabriel Mountains).
• The distribution of rocks of these ages suggests
  that the west coast of the North American
  Continent was well to the east of all but the
  southern end of what is now California.

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Proterozoic and Paleozoic Era

• All of these very old formations have been
  extensively metamorphosed - difficult to
  determine the conditions that existed when they
  were originally formed.
• Some of the oldest (around 1,800 million years
  old) are located in the mountains around Death
  Valley and are much like the rocks exposed in the
  inner gorge of the Grand Canyon.
• Metamorphic rocks around 1,000 million years old
  in the San Gabriel Mountains and the Orocopia
  Mountains east of the Salton Sea.

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Proterozoic and Paleozoic Era

• Between 1,000 million years ago and 400 million
  years ago was a quiet period in western North
  America.
• A passive (Atlantic type) margin
• The coastline remained east of California,
  probably in Utah and Idaho. Very thick sections
  of marine sedimentary rocks from this period are
  exposed in the mountains east of the Sierra
  Nevada.
• For the most part, these rocks are carbonates
  (limestone and dolomite), indicating shallow,
  warm ocean conditions with little incoming
  terrestrial sedimentpossibly similar to the Gulf
  of Mexico today.

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History of Californian continental margin

• Major events
• Rifting in the
• latest Pre-Cambrian
• Two orogenies, the
• Antler (Devonian) and
• the Sonoma (Permian
• Triassic) collisions of
• offshore volcanic arcs
• with North America

• Initiation of continental margin subduction with
  trench in todays western Sierra foothills
  (Triassic-Jurassic)
• Interruption of subduction by Late Jurassic
  Nevadan orogeny (accretion of another island arc
  terrane), initiation of Andean trench-gap-arc
  system at Franciscan-Great Valley-Sierra
  location
• Cenozoic subduction of Pacific-Farallon ridge
  leading to growing no-slab zone, Laramide orogeny
  in Rocky Mountains, then initiation of
  Basin-and-Range extension and San Andreas
  transform.

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Types continental margin

• California has been all four, at various times in
  the past billion years.
• Atlantic-type passive margin not a plate
  boundary
• Andean-type subduction close to shore, arc
  volcanoes on continental basement
• Japanese-type subduction offshore, marginal sea
  between the arc and the mainland
• Californian-type transform fault, no subduction,
  no spreading

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Antler and Sonoma Events

• Around 400 million years ago the quiescent period
  in western North America came to an end.
• Collision and accretion of arc terranes
• .

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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 granitic
  rocks that now make up most of the Sierra Nevada
  (subduction zone probably similar to the current
  situation along the West Coast of South America).

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Mesozoic Era 245-65 mya

• By 150 million years ago the Nevadan Mountains
  composed the western coast of North America and
  the sea never again extended farther east than
  the eastern edge of what is now the Central
  Valley.
• At roughly the same time the granitic rocks were
  forming that are now found in the Peninsular
  Range of Southern California and in the Salinian
  block that runs along the western edge of the San
  Andreas Fault in the central and southern Coast
  Ranges.

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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 million
  years 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
  to about the current location of the Coast Ranges
• Deposits in the subduction zone were mixed with
  rocks from the upper mantle beneath the crust
  producing the Franciscan Complex (extends from
  northern Santa Barbara to the northern California
  coast).
• The oceanic crustal material in the Franciscan
  formation was thrust up against the Sierran block
  by about 75 million years ago, extending the
  boundary of the North American plate further
  west.

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SUMMARY

• Accreted terranes
• Multiple orogenic events
• Transition from subduction to transform fault
• San Andreas Fault System
• Present uplift