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Faults
An intro to their geometries
Read text page 269-296
FAULTS
Faults - fractures with shear displacement along the surface.
Fault zones instead of just faults - quite common.
Recognizing faults:
1. Single faults surface:
Offset
Scarps (if they're young)
Slickensides and slickenlines with neomineral coating (occasionally)
Striations at larger scale are called "fault grooves"
Slickolites
2. Fault zones:
Closely spaced fault surfaces commonly separating masses of broken rock
Drag folding
Fault rocks: megabreccia- microbreccias and faults gouge , cat aclasites-ult racataclasite,
pseudotachylite
Map and surface expression of faults
.Geologic map expression. (note on strat concordant faults)
.Faults detected via subsurface drilling
.Faults detected by geophysical methods, gravity, magnetics, or
(most commonly) seismic methods
NAMING OF FAULTS
Slip (actual relative displacement) and separation (apparent relative displacement).
Stratigraphic throw.
Slip classification: faults can be: (1) strike slip,
(2) normal and (3) reverse faults as well as
combinations of the above. Thrust fault -= low angle (<30 degrees) reverse.
Hanging wall and footwall
Right-lateral and left lateral
Normal fault
Reverse fault
Strike-slip fault
What kind?
Complications: combinations of different fault types
Let’s try to name this one
Reverse left-slip fault
The overall stress framework for
faulting
• Reverse faults (thrust faults as a aspecial
low angle case) form in compression;
• Normal faults form during regional
extension
Let’s see how this works
Any fault related features?
Normal faults form a horst, in paleozoic sediments, in Death Valley
Small normal faults and. This is from Portovenere, Italy, near
Byron's Grotto..
Thrust fault
Aerial photo of San Andreas Fault showing distinct right lateral offset of streams.
Large scale grooving on a normal fault on
the north flank of the Coyote Mountains,
Salton Basin, CA. Fault surface dips 51° and
is dominantly dip slip with a small
component of left slip.
Bass Fault, Upper Granite Gorge, Grand Canyon. Precambrian rocks are on the left (west)
and lower Paleozoic rocks are on the right (east). Note, however, that the "drag" folds
indicate that the right side is down. This has been interpreted in terms of two directions of
movement, thrusting of the Precambrian over the Paleozoic and then reactivation of the
thrust as a normal fault.
The Solá fault strikes NW
and forms one of the
principal boundaries of the
Quebrada del Toro.
Important observation:
Reverse faults tend to put older rocks on top of
younger ones
Normal faults do the opposite
Note that this rule is not always applicable.