Home | Session 5 | California Geography Pg 1, 2

California Geography and Geology

Presented by Richard Sedlock
San Jose State University and BAESI

3. California's Tectonic Evolution (very abridged!)

Until about 250 Ma, the west coast of North America looked like modern east coast - no plate boundary, no earthquakes, no volcanoes. Evidence in California is found in the Paleozoic rocks (blue on CA geologic map) of eastern California: chiefly limestone, dolomite, shale, and other sedimentary rocks that were deposited in shallow water in areas that lacked much topography.

About 250 Ma, the world's plate motions underwent many changes, due chiefly to the formation of the supercontinent Pangea. Along the west coast of the Americas, subduction zones formed (see left, courtesy of USGS).

4. Effects on California (using the USGS' composite seismic section)
   
   
   1. We grew! All those Mesozoic rocks (green plus rocks under the yellow Central Valley) were added to the continent west of the older Paleozoic (blue) rocks.
      
      
   2. The Sierra Nevada-Peninsular Ranges batholith was constructed, and was overlain by a volcanic arc that's been almost completely eroded away.
      
      
   3. Just east of the trench, various rocks were added to North America where the two plates met. This is the famed Franciscan Complex (darker green on the map). Franciscan rocks include:

      basalt - volcanic rocks that formed at spreading centers or as volcanic islands
      chert - deep-ocean sediment that was deposited atop basalt
      limestone - shallow-ocean sediment that capped volcanic seamounts
      sandstone & conglomerate - washed down from the Sierran arc

      Parts of the Franciscan, these (and other) rocks are scrambled together to form mélange.

Starting 28 Ma in soCal (about 10 Ma here): transform replaced subduction. See a Quicktime animation by Tanya Atwater: (http://www.geol.ucsb.edu/%7Eatwater/Animations/Animations-FR.html, scroll down to #A2 and follow download instructions)

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updated March 4, 2002

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