CALIFORNIA – GEOLOGIC HISTORY AND
PLATE TECTONICS
VICKI DRAKE
EARTH SCIENCE DEPARTMENT
SANTA MONICA COLLEGE
I. California’s history begins ~1 billion years ago
A. Geologic movements developing great North American Cordillera
1. Mountain chain extending from Alaska to Guatemala
II. Plate tectonics
A. A general theory describing movements of continental and oceanic crustal plates
B. Earth’s Lithosphere (composed of crust and upper mantle materials) broken into large ‘plates’ that move round the Earth
C. Over billions of years, these plates tend to move, forming, breaking, and reforming continental landmasses
D. Continental landmasses of today took shape ~ 250-200 millions years ago
E. As boundaries of tectonic plates, various interactions are possible
1. Convergent boundaries (two plates moving towards each other)
a. Oceanic-oceanic: Subduction, volcanism (island arc), deep earthquakes, deep ocean trenches
b. Oceanic-continental: subduction, volcanism (continental arc), deep earthquakes, deep ocean trenches
c. Continental-continental: No subduction, no volcanism, mountain-building, shallow earthquakes
2. Divergent boundaries (plate splitting into two pieces moving away from each other)
a. Mid-Ocean Ridge: Volcanism, mantle material forming new ocean floor, mountain building (chain around Earth), earthquakes
b. Rift Valleys: East African Rift Valley, elongated lakes, opening of new oceans (Red Sea)
3. Transform boundaries: (two plates sliding past each other, horizontally)
a. Folding and faulting of crust as rock formations are ‘pulled’ . Shearing forces
b. Earthquakes, displaced streams, linear troughs of groundwater at surface (sag ponds)
III. California’s geological development
A. Interaction between two major tectonic plates: Pacific Plate and North American Plate
B. California part of “Pacific Ring of Fire”: collection of active volcanism and earthquakes encircling Pacific Ocean basin
IV. Time Frame for California’s development
A. Pre-Cambrian (~600 mya) to early Paleozoic (~400 mya) California covered by shallow sea
B. Volcanic activity offshore (due to subduction of Pacific Plate beneath North American Plate) created lava flows, ash falls accumulating on sea floor
C. Sediment material washing off continents accumulating on sea floor
D. Mesozoic era (~200 mya), volcanic and sedimentary material ‘squeezed’ up off sea floor during Nevada orogeny
1. First building of ancient Sierra Nevadas, Klamath Mountains, and Peninsular Ranges
a. ~150 mya-140 mya subduction process slowed as tectonic boundary between Pacific/North American plates collapsed
b. Western edge of continent and shallow sea floor folded, tilted to west.
c. Eastward, series of normal faults formed ancient Basin and Range province (Eastern California)
2. Renewal of subduction after this time: magmatic material ‘emplaced’ beneath continental crust
a. Beginning of modern Sierra Nevada Range: granitic (granodiorites) rock batholith
b. Process of emplacement between ~140 mya – 90 mya.
3. West of Sierra Nevada emplacement activities
a. Shallow trough or depression forming in sea, filling with sediment, ocean material
b. Trough eventually filled in to form Sacramento and San Joaquin Valleys
c. Materials ‘scraped’ off ocean floor during initial building forms Coastal Ranges (extending from California through Pacific Northwest)
E. Cenozoic Era (~ 30 mya): spreading zone in Pacific Plate (East Pacific Rise) arrives at western edge of ancient “California”
1. Subduction in southern California ceases at this time
2. Pacific Plate re-orients itself and detaches portions of western/southern California from North American Plate
3. New boundary formed: Transform Plate Boundary (a lithospheric fault zone: San Andreas Fault)
4. Transverse Ranges rotated from N/W trending to more E/W trending (process began ~65 mya)
5. Portions of Southern, Central and Northern California now part of a pull-apart faulting system (San Andreas) moving in a northwesterly direction.
6. San Andreas Fault not perfectly aligned
a. Fault ‘bends’ to west around edge of Transverse Ranges
b. Mojave Desert compressed and squeezed
c. Coastal Mountains still rising due to folding/faulting along San Andreas Fault
7. Klamath Mountains (originally part of Sierra Nevada Range) moved westerly as Pacific Plate still subducted in northern California
8. Modoc Plateau and Cascade Volcanic Mountains formed in recent times
a. Volcanism still possible here
b. Mt. Lassen and Mt Shasta potentially dangerous
9. Basin and Range Province, created by crustal extension
a. Death Valley, Panamint Valley, Saline Valley, Owens Valley, etc.
b. White-Inyo Mountains, Panamint Mountains, Amargosa Mountains, Last Chance Ranges, etc.
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© Vicki Drake
SMC