Access to this location is either from the
South Fork Camp from the east or from Devil’s Punchbowl County
Park from the west. In either case it is a moderate to
strenuous hike. The last few hundred feet follows a
geologist’s trail to a narrow ridge next to a steep cliff. Use
the provided waypoints for guidance on how to get from the
trail to the earthcache. Still use your own best judgment in
getting to the final coordinates.Be sure to be prepared for
the weather and carry enough food and water.
The Punchbowl Fault is the daddy of the San Andreas Fault since
it is considered to be an abandoned branch of the more famous
currently active fault. After this fault was abandoned, the area
has been uplifted and eroded down to expose the part of the fault
that was once deep underground. This allows us to infer not only
what is currently occurring deep underground along the San Andreas
since these two faults are part of the same fault system but also
gives us a glimpse at the contact between plates since the San
Andreas is the boundary between the North American and Pacific
Plates.
As you leave the trail notice the conglomerate outcrops of the
sedimentary Punchbowl Formation. Further up the mountain are the
metamorphic and igneous (together referred to as crystalline) rocks
of the San Gabriel Mountains. Once you reach the coordinates you
will find a two linear zones of pulverized rock, one black and the
other tan, between the sedimentary and crystalline rocks that runs
roughly northwest/south east. This is the Punchbowl Fault.
This pulverized rock is called ultracataclasite.
Ultracataclasite is a metamorphic rock that forms from pulverized
rock under high temperature and pressure as would be expected where
two plates are sliding past each other and deep in the earth. This
type of metamorphism is called dynamic metamorphism because it is
caused by action.
The black ultracataclasite formed from the crystalline rock
while the tan ultracataclasite formed from the sedimentary rock,
and there is little mixing of the two. As you move further away
from the fault, there is a sharp boundary between the
ultracataclasite and severely fractured but unaltered rock. The
amount of fracturing quickly decreases away from the fault.
Chester, Fredrick M. et al 2004
Based in part on the lack of mixing of the two colors of
ultracataclasite and evidence beyond the scope of this description,
Frederick Chester of Texas A&M University suggests that all
movement along the fault occurred along the line between the two
colors of ultracataclasite and thus infers that this same process
is occurring along the San Andreas Fault allowing the Pacific and
North American Plates to slide past eachother.
Logging requirements:
Send me a note with :
- The text "GC1AAAP The Punchbowl Fault - View a Plate Boundary"
on the first line
- The number of people in your group.
- At this location, how wide is the ultracataclasite layer
- Do you agree that the black and tan ultracataclasite layers
don’t mix
- Is the contact between the ultracataclasite layers and the
crystalline or sedimentary rock linear, sinuous, or
chaotic?
The above information was compiled from the
following sources:
- Chester, Fredrick M. 1999, Field Guide to the
Punchbowl Fault Zone at Devils Punchbowl Los Angels County Park,
California, V. 2.1, January 1999 Center for Tectonophysics, Dept.
of Geology and Geophysics, Texas A& M University, College
Station, TX
- Chester, Fredrick M. et al 2004, Structure of
large-displacement, strike-slip fault zones in the brittle
continental crust In: Rheology and Deformation in the Lithosphere
at Continental Margins, Edited by Karner, G. D., B. Taylor, N. W.
Driscoll, and D. L. Kohlstedt, Columbia University Press, New York,
2004.
- John W. Shervais Metamorphic Petrology lecture
notes (?) Utah State University, Department of Geology
http://www.usu.edu/geo/shervais/G4500_PDF/METAMORPHIC%20PETROLOGY.pdf