Download Powerpoint

Today’s list____________
Ch15: Rock Deformation
1)Mapping geologic structures
2)How rocks get deformed
3) Folds
Study questions____________
Ch15: Rock Deformation
·
What type of tectonic forces makes a normal fault?
A reverse fault? A strike slip fault?
What is the difference between brittle and ductile
deformation?
What are the strike and dip angle of a deformation?
What is the shape of an anticline, syncline, and
what is the relative age of rocks exposed?
What type of deformation is the San Andreas Fault?
The mid-ocean ridge?
1) Mapping geologic structures
Deformational processes create faults, folds, joints
rock structures
Visible when rock beds are exposed at surface
out crops
1) Mapping geologic structures
 Strike and dip
TSP 15.8
1) Mapping geologic structures
 Strike and dip
Strike
Compass bearing of a rock layer
relative to North
Dip
Angle of tilt of a bed from a horizontal
plane
Fig.11.4
1) Mapping geologic structures
N 75 E
2) How rocks become deformed
Confining pressure: force equal in all directions
Differential stress: force not equal in all directions
2) How rocks become deformed
Response to differential stress
1. Elastic deformation – the rock returns to nearly its original size and
shape when the stress is removed
2. Brittle deformation
rocks crack/fracture
Shallow in crust
3. Ductile deformation
rocks flow
Deeper down
2) How rocks become deformed
Fault definition: a fracture where displacement has occurred:
rocks on either side of fault have moved relative to each other.
2) How rocks become deformed
Fault definition: a fracture where displacement has occurred:
rocks on either side of fault have moved relative to each other.
Joints
Fractures where no
slip has occurred
Not same as a fault!
Why?
Slip happens on faults!
2) How rocks become deformed- brittle materials
Differential stress: force not equal in all directions
Compressive forces:
shortening
2) How rocks become deformed- brittle materials
Differential stress: force not equal in all directions
Tensional forces:
stretching
2) How rocks become deformed- brittle materials
Differential stress: force not equal in all directions
Shearing forces:
shearing
2) How rocks become deformed - brittle materials
2) How rocks become deformed - brittle materials
Fig. 11.13
Rift valleys are the result of tensional forces in
the crust, creating a series of normal faults.
The downfaulted blocks result in a Rift Valley.
2) How rocks become deformed - brittle materials
St. Andreas fault is an
example for a strike-slip
fault
Because it is a plate
boundary, it is also a
transform fault
Fig. 11.10
Fault System Example : San Andreas
2) How rocks become deformed- ductile materials
Differential stress: force not equal in all directions
Compressive forces: Tensional forces:
shortening
stretching
Shearing forces:
Shearing/bending
3) How rocks fold
 series of wavelike undulations of once-flat rock layers
 many sizes: microscopic to 100’s of meters
compressive stresses: shorten+thicken crust
3) Folds
Anticlines fold upward… Synclines fold downward…
Fig. Story 11.16
3) Folds
horizontal and plunging
anticline
Plunging fold:
axis of fold
penetrates into
the ground
Fig. 11.16
Plunging anticlines and
synclines
See Fig. 11.17
3) Folds
 Types of folds
Symmetrical, asymmetrical, overturning,
plunging…..
 Other deformations
dome - circular
upwarping (anticlinal
structure)
basin - circular
downwarping (synclinal
structure)
Fig 11.19