This requires a decent hike in from either the Devil’s
Punchbowl County park or South Fork Campground. The examples
listed here are just off the trail along hard rock.
The nearby San Andreas and the abandoned Punchbowl fault are
both strike-slip faults. Movement on strike slip faults is almost
all horizontal with little vertical movement. These kinds of faults
are described as either sinistral (left-lateral) or dextral
(right-lateral) depending upon how the rock blocks move relative to
each other. For sinistral faults, the rock on the other side of the
fault appears to have moved left relative to first block. And for a
dextral fault, the rock on the other side of the fault appears to
have moved right.
What does that actually mean? Imagine you were watching a tree on
the other side of a strike-slip fault during an earthquake. If the
fault was sinistral, at the end of the earthquake, the tree would
appear to have moved further to your left and if it was dextral,
the tree would appear to have moved further to your right. Notice,
it doesn’t matter which side of the fault you are on, the other
side of the fault will always have the same relative movement. The
diagram is from the viewpoint of a balloon above the fault.
This is a rather dry textbook description and description. Now
how do you actually recognize or identify this movement in the
field? The coordinates bring you to an outcrop where you can find
small strike-slip faults of both styles. You are looking for two
8-inche diameter rocks, one red and black and one olive tan. They
are within about 3 feet of each other. If you are having trouble
finding them, they are located beneath my sunglasses and gps in the
attached picture. Each rock has a strike-slip fault that has sliced
the rock into two pieces. One fault is sinistral and the other is
dextral.
Stand on one side of the fault, it doesn’t matter which side.
Imagine the rock as a single piece and then imagine which way the
rock on the other side of the fault would have to move to get to
its current position. This method can be used on any size fault to
determine the style of strike slip faulting, even one as large as a
plate boundary such as the San Andreas Fault.
The secondary coordinates take you to another style of faulting,
called either normal or reverse (thrust) faulting. Movement along
this kind of fault is primarily vertical, one side will move up or
down relative to the other. These faults are usually slanted to
some degree so you can picture the fault as a hill. In a normal
fault, the rocks appear to slide down the fault as you would expect
gravity to pull something down the hill. In a reverse or thrust
fault, the rocks appear to have been pushed up the hill.
Normal faulting is characteristic of rocks being pulled apart.
As the rocks pull away from each other, material falls down to fill
in the space created. Reverse faulting is characteristic of
compression. Rocks being pushed together will pile up on top of
eachother.
Logging requirements:
Send me a note with :
- The text "GC1AH5X Which way the Fault Goes?" on the first
line
- The number of people in your group.
- The color of each rock and the style of strike-slip fault that
cuts it.
- Is the style of faulting at the secondary coordinates normal or
reverse?
The above information was compiled from the
following sources:
- Fault (geology), Wikipedia, the free
encyclopedia
http://en.wikipedia.org/wiki/Geologic_fault
More detail
Many minor faults are generated adjacent to a primary fault. These
minor faults form to release the stress and strain created by the
primary fault. These minor faults do not have to have the same
style of faulting as the primary fault.