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Folds and Faults
Stress
Stress is the force applied to rocks due
to moving of the Earth’s plates.
There are 3 types of stress:



Compression – occurs at converging plates;
rocks generally thicken and shorten
Tension – occurs at diverging plates; rocks
generally thin and lengthen
Shearing – occurs at transform plate
boundaries; rocks are sliced into parallel
blocks
Stress and Strain
When rocks are subjected to stress,
they either bend (fold) or break (fault).
Strain is the deformation of rock as a
result of particular forces applied to the
rock.
Stages of Deformation
When a rock is subjected to increasing stress
it passes through three successive stages of
deformation.
Elastic Deformation - the strain is reversible
Ductile Deformation - the strain is irreversible
Fracture - irreversible strain in which the
material breaks
Folds
Rocks fold due to compression at
convergent plates.
The most common types of folds are
tightly folded arches and troughs.
The arches are called anticlines.
The troughs are called synclines.
Anticlines and Synclines
anticline
syncline
A dome is an up-arched series of strata with beds on all sides
dipping away from the center.
An eroded surface indicates the rocks become progressively
younger away from the center of the structure
A basin is a down-arched series of strata with beds on all sides
dipping in towards the center
An eroded surface indicates the rocks become progressively
older away from the center of the structure.
Faults
A fault is a break in a rock where the
rocks on either side of the crack slide
past each other.
There are 2 main types of faults:


Strike-slip faults
Dip-slip faults
Strike-slip Faults
Occur at transform plate boundaries
where the fault blocks (rocks on either
side of the crack) move horizontally
past each other.
Ex. the 1000 km long San Andreas fault
in California.
San Andreas
Fault
Dip-slip Faults
In dip-slip faults the fault blocks move
vertically past one another.
The block above the fault is called the
hanging wall.
The block below the fault is called the
footwall.
Dip-Slip Faults
footwall
hanging
wall
Dip-slip Faults, cont’d
There are 2 types of dip-slip faults:


Normal dip-slip – the hanging wall moves
downward. This is associated with
tensional stress.
Reverse dip-slip – the hanging wall moves
upward. This is associated with powerful
compression.
Normal Dip-slip
Reverse Dip-slip
Normal or Reverse?
Summary Table
Fault Type
Stress
Plate Boundary
strike-slip
shearing
transform
normal dip-slip tension
diverging
reverse dip-slip compression
converging