Tectonic Evolution of California

I have to admit from the beginning that I'm not a geologist, so I may well make a mistake or two in this section. I apologize in advance for any such problems you might find. If you do find one, please send me e-mail so that I can fix it.

Very little of what is now California was originally a part of North America back in the Precambrian Era; basically the Mojave Desert basement rocks were around then. Most of the rest of California is made up of large chunks of different kinds of rocks which were ``imported'' from other parts of the world through plate tectonic movements. These big chunks of rock are called displaced terranes.

California is an amazing mess of such terranes, and doing a good job of giving the full history of these terranes would certainly take a full book. So I'm not even going to try. Instead, I'll give a brief timeline in list form, with the main highlights of California's evolution outlined. At the end of this section, I'll try to list some other good references to look at for more detailed information. For reference, ``Ma'' means ``million years ago''.

• Precambrian time (2,000-540 Ma)
  • Outcrops in the Mojave Desert, San Bernardino and San Gabriel Mountains.
  • Oldest rocks in California, about 1,800-2,000 million years old.
• Early to Mid-Palaeozoic/Cambrian to Mid-Devonian (540-400 Ma):
  • West coast very similar to East coast today.
  • Passive continental margin with no subduction.
  • Rocks from this age are found in the Inyo Mountains of eastern California and in the Great Basin area.
• Mid-Palaeozoic to Early Mesozoic/Mid-Devonian to early Triassic (400-240 Ma):
  • Transition from a passive continental margin to a subduction zone.
  • Island arc volcanism and thrusting of continental shelf deposits from earlier passive margin.
  • Accretion of various displaced terranes.
  • Island arcs, ophiolites, and mélanges accreted to form ancestral Klamath Mountains and Sierra Nevada.
  • Rocks from this age form roof pendants (sort of a giant hat) over later batholiths. The pendants are found in the Sierra near Convict Lake, among other places. Also form much of the Basin and Range province.
• Early Mesozoic/Mid-Triassic (240-215 Ma):
  • Spreading center hits continental boundary.
  • The Foothill suture zone (west side of Sierra) forms.
• Mesozoic to Mid-Cenozoic/Late Triassic to Mid-Oligocene (210-30 Ma):
  • Farallon Plate subducts under W coast of North America
  • Central Valley is a forearc basin, gets filled with submarine sediments and turbidites from ancestral Sierra Nevada.
  • Great Valley sequence (forearc basin sediments) accreted
  • Coast Range Ophiolite (ophiolites are believed to be chunks of ocean floor) squished onto the underside leading edge of the North American Plate (a process called underplating).
  • Franciscan sequence (a huge mess of multiple terranes) is underplated
  • Sierra Nevada and Peninsular Range batholiths are formed. Peninsular Range mountains are Baja and San Jacintos, etc.
  • Salinian block formed. Either formed in its present location (west-central CA) or formed much farther away.
• Oligocene (30 Ma):
  • Farallon-Pacific spreading center hits coast
  • Triple junctions form. One migrates north (Mendocino TJ) and other south (Rivera TJ).
  • Ancestral San Andreas Fault forms
• Miocene (10-5 Ma):
  • Rivera TJ jumps inland, perhaps several times
  • This might create several faults in southern California (i.e. Elsinore)
• Miocene-Pliocene (5 Ma):
  • Gulf of California opens
  • Big Bend in SAF forms
  • Los Angeles, Santa Barbara, and Ventura Basins form, fill in with sediments.
  • Uplift of mountains north of Los Angeles
• Mid-Pliocene (2 Ma):
  • Southernmost SAF forms in ``present'' position
• Pleistocene-Present (1.6-0 Ma):
  • Ice Ages
  • Humungous lakes form, such as Lake Bonneville (the much much bigger version of the Great Salt Lake), Lake Lahontan (which contained what are now Honey Lake, Walker Lake, Pyramid Lake and the Carson Sink in Nevada, among other things), and Lake Cahuilla (the much bigger version of the Salton Sea). Then they shrink, and form, and shrink, and form, and shrink...
  • Glaciers in the Sierra
  • Long Valley Caldera and other volcanism near Great Basin

Please keep in mind that this is an extremely bare-bones chronology and no doubt I have left out lots of stuff.

Here's another list of odd or otherwise interesting facts I dug up in the process of making the list above:

• Franciscan rocks have moved 2000 km (based on palaeomagnetism)
• Franciscan rocks make up both much of NW CA and Channel Is
• Coast Range ophiolite moved 2-4000 km (based on palaeomagnetism)
• Salinian block either part of Sierra or displaced terrane from 2500 km away
• Up to 560 km right-lateral slip on SAF (demonstrated by mismatched rocks)
• LA Basin sediments up to 27000 feet (8300 meters) thick

Figure 1 is a nice picture from the USGS which shows the last 30 million years of California's evolution.

  Figure 1. Schematic diagram showing the last 30 million years of evolution of the California continental margin. From USGS Professional Paper 1515.

 

Here are a couple places to look for more information. Both are fairly technical.

• The Geotectonic Evolution of California, W.G. Ernst, editor. Prentice-Hall, Englewood Cliffs, N.J. 1981
• The San Andreas Fault System, California, Robert E. Wallace, editor. US Geological Survey Professional Paper 1515, 1990.

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