I
Deformation
A. General term that refers to all changes in the original form of a body of rock
B. Most crustal deformation occurs along plate margins
C. Involves
1) Force
2) Stress-amount of force applied to
given area
i. Confining stress-uniformly
distributed
ii. Differential Stress- unequal
distribution of force
(a) Compressional Stress –results
in shortening of crust
(b) Tensional stress- result in
lengthening of rock body
(c) Shear stress
iii. Strain- irreversible change in
shape and size of rock body caused
by stress
D. How deformation occurs
1) Rocks subjected to stress greater than
their strength deform by:
i. Folding
ii. Flowing
iii. Fracturing
2) Factors that influence deformation
i. Temp
ii. Confining pressure
iii. Rock type
iv. Time
II Mapping Structures
A. Usually only limited number of outcrops available to study.
1) Study is aided by aerial photos
2) Satellite imagery
3) GPS
B. Describing and mapping structures
1) Strike-compass direction of line
created by the intersection of
imaginary horizontal plane with
inclined layer of rock or fault line
i. Generally expressed as angle
relative to north
2) Dip- angle of inclination of layer
of rock or fault line relative to
imaginary horizontal plane
i. Include both angle of inclination and direction of inclination
III Folds
A. Most fold result from compressional stress
B. Parts of fold
1) Limbs- refers to the two sides of a fold
2) Axis- line drawn down the points of maximum
curvature (the top of the arc or bottom of trough)
3) Axial Plane- imaginary surface that divides a fold
symmetrically
C. Types of folds
1) Anticline-upfolded layer of rock
2) Syncline- downfolded layer of rock
i. Depending on orientation and stress involved in
deformation anticlines and synclines can be
(a) Symmetrical
(b) Asymmetrical
(c) Recumbant (overturned)
3) Monoclines- large step-like folds in
otherwise horizontal strata
4) Other types of deformation
i. Domes-circular or slightly
enlongated, upward displaced
strata
ii. Basin- circular or slightly
enlongated, upward displaced
strata
IV Faults
A. Faults are natural fractures within bodies of rocks along which considerable
displacement can occur
1) Movement often results in earthquakes
B. Classified by relative movement
1) Horizontal
2) Vertical
3) Oblique
C. Types of Faults
1) Dip-slip fault
i. movement along fault is primarily parallel to the dip of the fault surface
ii. May produce long low cliffs known commonly as scarfs
iii. Part of dip-slip fault
(a) Hanging wall- the body of rock
that makes up the ceiling of the
fault (the side that is on top of the
fault)
(b) Footwall- the body of rock that
makes up the floor of the fault (the
side that is on the bottom of the
fault)
iv. Types of dip-slip faults
(a) Normal Fault
(i) hanging wall moves
down relative to footwall
(ii) accommodates
lengthening of the crust
(iii)most are relatively small
(iv) Large scale normal faults
associated with fault-block mountains
(b) Reverse Fault
(i) hanging wall moves
upward relative to
footwall
(ii) accommodates
shortening of the crust
1. reverse faults have
angle greater than
45
2. thrust faults have angle less than 45
(c) Strike-Slip Fault
(i) Dominant displacement is horizontal and parallel to the strike of
the fault
(ii) Types of strike-slip faults
1. Right lateral- as you face the fault the opposite side appears to
move to the right
2. Left lateral- as you face the fault the opposite side appears to
move to the left
3. Transform fault- large strike-slip fault that cuts through the
lithosphere and forms the boundary between two lithospheric
plates.