Geologic framework and history of the Santa Teresa Hills
Geologic framework and history of the Santa Teresa Hills
Geologic framework and history of the Santa Teresa Hills
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<strong>Geologic</strong> <strong>framework</strong><br />
<strong>and</strong> <strong>history</strong> <strong>of</strong> <strong>the</strong><br />
<strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong><br />
Richard Sedlock, SJSU Geology
Major Points<br />
• Rocks in <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong> record major<br />
events in California’s geologic <strong>history</strong>.<br />
• Serpentinite (<strong>the</strong> “rocky unit”) <strong>and</strong> mélange<br />
(<strong>the</strong> “grassy unit”) formed in a subduction<br />
zone in <strong>the</strong> Jurassic <strong>and</strong> Cretaceous.<br />
• They are in contact at a fault that (1) is no<br />
longer active, <strong>and</strong> (2) has a SHALLOW<br />
inclination, contrary to most published maps.
Major Points, cont.<br />
• Tertiary sedimentary rocks sit on top <strong>of</strong> <strong>the</strong><br />
older serpentinite <strong>and</strong> mélange.<br />
• The plate boundary in California changed<br />
from a subduction zone to a transform fault<br />
about 15 million years ago.<br />
• The <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong> were raised in <strong>the</strong> last<br />
several million years, due partly to <strong>the</strong><br />
dem<strong>and</strong>s <strong>of</strong> a bend in <strong>the</strong> San Andreas fault.
Earth’s tectonic plates
Types <strong>of</strong> plate boundaries
A typical subduction zone
<strong>Geologic</strong><br />
map <strong>of</strong><br />
California
Old California:<br />
The subduction zone<br />
• The cross section is a “Frankenstein” image<br />
that shows seismic reflection data collected<br />
at many places around <strong>the</strong> modern world.<br />
• U.S. <strong>Geologic</strong>al Survey personnel spliced<br />
<strong>the</strong>m toge<strong>the</strong>r to illustrate a probable eastwest<br />
cross section <strong>of</strong> California in <strong>the</strong> mid-<br />
Cretaceous (about 100 million years ago).
Subduction-zone rock units<br />
in <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong><br />
• Serpentinite is metamorphosed mantle. It is<br />
<strong>the</strong> “rocky unit.”<br />
• Mélange is a mixture <strong>of</strong> isolated blocks in a<br />
matrix <strong>of</strong> sheared s<strong>and</strong>stone <strong>and</strong> shale. It is<br />
<strong>the</strong> “grassy unit.”
<strong>Geologic</strong> map <strong>of</strong> part <strong>of</strong> <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong>
<strong>Geologic</strong> map <strong>of</strong> part <strong>of</strong> <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong>
<strong>Geologic</strong> map <strong>of</strong> part <strong>of</strong> <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong>
Cross-section <strong>of</strong> <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong><br />
(USGS) (SW-NE, through Stile Ranch)
Cross-section <strong>of</strong> <strong>the</strong> <strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong><br />
(Sedlock/SJSU) (SW-NE, through Stile Ranch)<br />
serpentinite<br />
Tertiary<br />
sedimentary rocks<br />
serpentinite<br />
Franciscan mélange with exotic blocks
Q: Whence <strong>the</strong> serpentinite<br />
Q: How were serpentinite & mélange juxtaposed
Mt. Hamilton<br />
Calaveras fault<br />
Calaveras fault<br />
serpentinite
Movie won’t play in<br />
PDF format; obtain<br />
separate QuickTime file
Modern<br />
tectonic<br />
setting
<strong>Santa</strong> <strong>Teresa</strong> <strong>Hills</strong><br />
Satellite image <strong>of</strong> <strong>the</strong> Bay Area<br />
Active faults shown in red
Note <strong>the</strong> bend in <strong>the</strong> San Andreas fault
Conclusions<br />
• The hills <strong>of</strong> <strong>the</strong> south Bay probably were raised<br />
in <strong>the</strong> last few million years.<br />
• The likely cause is <strong>the</strong> bend in <strong>the</strong> San Andreas<br />
fault, which causes thrust faulting, folding, <strong>and</strong><br />
crustal uplift.<br />
• The chief architect <strong>of</strong> <strong>the</strong> Bay Area, <strong>and</strong> <strong>the</strong> bane<br />
<strong>of</strong> its urbanized human population, is strike-slip<br />
faulting <strong>and</strong> its accompanying earthquakes.
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