1. CRUSTAL DEFORMATION
and Geologic Structures

2. Deformation
• Deformation involves:
– Stress – the amount of force applied to
a given area.
– Types of Stress:
–Confining Stress – stress applied
equally in all directions.
–Differential Stress – stress applied
unequally in different directions.

3. Deformational Stress
• Types of Differential Stress:
(1) Compressional Stress – shortens and thickens a
rock body (associated with convergent plate
boundaries).
(2) Tensional Stress – tends to elongate and thin or
pull apart a rock unit (associated with divergent
plate boundaries).
(3) Shear Stress – produces a
motion similar to slippage that
occurs between individual
playing cards when the top of
the stack is moved relative to
the bottom (associated with
transform plate boundaries).

4. Deformation
of the Earth’s
Crust Caused
by Tectonic
Forces and
Associated
Differential
Stresses

5. Deformation
• Differential stress applied to rocks during
tectonic activity causes rocks to respond via
deformation.
• Strain – changes in the shape or size of a rock
body caused by stress.
• Strained rock bodies do not retain their original
configuration during deformation.

6. How Do Rocks Deform?
• Rocks subjected to stresses greater than
their own strength begin to deform
usually by folding, flowing, or
fracturing.
– General Characteristics of Rock
Deformation:
• Elastic deformation – the rock returns to
nearly its original size and shape when the
stress is removed.
• Once the elastic limit (strength) of a rock is
surpassed, it either flows (ductile deformation)
or fractures (brittle deformation).

7. • Factors that influence the strength
of a rock and how it will deform:
• Depth
• Temperature
• Confining Pressure
• Rock Type
• Availability of Fluids
• Time
How Do Rocks Deform?

8. • Rocks near the surface, where confining pressures and
temperatures are low, will behave as a brittle solid and
fracture once their strength is exceeded.
• Rocks at depth, where confining pressures and
temperatures are high, will exhibit ductile behavior or
solid-state flow, in which changes occur without
fracturing.
How Do Rocks Deform?

9. Crustal Structures
• Folds – During crustal deformation rocks
are often bent into a series of wave-like
undulations.
– Anticlines and Synclines
– Domes and Basins
– Monoclines
• Characteristics of Folds:
• Most folds result from compressional
stresses which shorten and thicken the crust.
• Most of them occur in a series.

10. Anatomy of a Fold
• Limbs – Refers to the two sides of a fold.
• Axis (or Hinge) – A line drawn down the
points of maximum curvature of each
layer.• Axial Plane – An
imaginary surface
that divides a fold
symmetrically.
• Plunge – In
complex folding,
the axis is often
inclined at an angle
called plunge.

11. (A) Horizontal
Anticline
and
(B) Plunging
Anticline

12. Common Types of Folds
• Anticline – upfolded
or arched rock
layers.
• Syncline –
downfolds or
troughs of rock
layers.
Photo courtesy of J. T. Daniels
http://disc.gsfc.nasa.gov/geomorphology/GEO_2/GEO_PLATE_T-42.shtml
Photo courtesy of Brennan T. Jordan, Department of Earth Sciences,
University of South Dakota
http://www.usd.edu/~Brennan.Jordan/

13. Common Types of Folds
• Depending on their orientation, anticlines and
synclines can be described as…
• Symmetrical, asymmetrical, overturned, recumbent
(a type of overturned fold – “lying on its side”), or
plunging.

14. Formation
of Folds
Insert Animation #30:
Folds

15. Name the Folds Below

16. Plunging
Anticlines
and
Synclines
(Note: the outcrop
pattern of an
anticline points in
the direction it is
plunging, whereas
the opposite is
true for a
syncline)

17. Sheep Mountain, A Plunging Anticline

18. Formation
of Folds
Insert Animation #30:
Plunging Folds

19. Other Types of Folds
• Monoclines
• Large, step-like folds in otherwise horizontal sedimentary strata.
• Closely associated with faulting.

20. Other Types of Folds
• Dome
• Upwarped displacement of rocks.
• Circular or slightly elongated structure.
• Oldest rocks in center, younger rocks on the flanks.

21. Other Types of Folds
• Basin
• Circular or slightly elongated structure.
• Downwarped displacement of rocks.
• Youngest rocks are found near the center, oldest
rocks on the flanks.

22. Crustal Structures
• Faults – Fractures in rocks along which appreciable
displacement has taken place.
• Fault Zone – Displacements along multiple interconnected
faults.
• Sudden movements along faults are the cause of most
earthquakes.

23. Types of Faults
• Classified by their relative movement
which can be Horizontal, Vertical, or
Oblique.

24. Summary of Fault Types
• Dip-Slip Faults:
• Normal (gravity) – associated with divergent plate
boundaries.
• Reverse and Thrust – associated with convergent
plate boundaries.
• Strike-Slip Faults:
• Lateral (right and left) – associated with transform
plate boundaries.

25. Dip-Slip Faults
• Movement is
mainly parallel to
the dip of the
fault surface.
• Parts of a dip-slip
fault include the
hanging wall
(rock surface
above the fault)
and the footwall
(rock surface
below the fault).

26. Dip-Slip Faults
• Normal Fault (gravity)
Dip-Slip Faults
– Hanging wall block moves
down relative to the footwall
block.
– Tensional stress
– Accommodate lengthening or
extension and thinning of the
crust.
– Associated with divergent plate
boundaries.
– Most are small with
displacements of a meter or so.
– Larger scale normal faults are
associated with structures
called fault-block mountains
(Teton Range in Wyoming,
Basin and Range Province in
Nevada).

27. Formation
of Normal
Faults
Insert Animation #29:
Faults – Normal

28. Normal Faulting – Fault Block Mountains
• Fault-Block Mountains – Basin and Range Province
in Nevada – topography generated by a system of
roughly north to south trending normal faults.
• Movements along these faults have
produced alternating uplifted blocks
called horsts (form elevated ranges)
and down-dropped blocks called
grabens (form basins).
• Half-Grabens – a tilted fault block in
which the higher side forms
mountainous topography and the
lower side forms a basin that fills
with sediment.
• Detachment Fault – nearly horizontal
fault extending up to hundreds of
kilometers into the subsurface.
Smaller faults are connected to this
larger fault. Boundary between
ductile and brittle deformation.

29. Dip-Slip Faults
• Reverse and Thrust Dip-Slip Faults
– Hanging wall block moves up relative to the
footwall block.
– Reverse faults have dips greater than 45o
– Thrust faults have dips less than 45o
.
• Strong compressional stress.
• Accommodate shortening
and thickening of the crust.
• Associated with convergent
plate boundaries.

30. Formation
of Reverse
Faults
Insert Animation #29:
Faults – Reverse

31. Idealized Development of Lewis Overthrust
Fault near Glacier National Park

32. Strike-Slip Faults
• Dominant displacement is horizontal
and parallel to the strike of the fault.
• May produce broad zones of roughly
parallel fractures up too several
kilometers in width.
• Shear stress.
• Associated with transform plate
boundaries.

33. Formation
of Strike-Slip
Faults
Insert Animation #29:
Faults – Strike-Slip

34. Types of Strike-Slip Faults
• Right-Lateral – as you face the fault, the
opposite side of the fault moves to the right.
• Left-Lateral – as you face the fault, the
opposite side of the fault moves to the left.
http://www.pbs.org/wnet/savageearth/animations/
Animations:
Right-Lateral
Strike-Slip Fault

35. Types of Strike-Slip Faults
• Transform Fault
– Large strike-slip
fault that cuts
through
accommodates
motion between
two large crustal
plates.
– Example: San
Andreas Fault
System

36. Name the Type of Fault Below

37. Name the Type of Fault Below

38. Name the Type of Fault Below
Insert Animation #28:
Exposing Metamorphic Rock

39. Name the Type of Fault Below

40. Name the Type
of Fault Below

41. Name the
Structure

42. Name the
Structure

43. Mapping Geologic Structures
• Geologists measure the orientation or
attitude of a rock layers or
fault/fracture surfaces in order to
describe and map geologic structures
that result from deformation.

44. Mapping Geologic Structures
– Strike (Trend)
• The compass direction of the line produced by the
intersection of an inclined rock layer or fault with a
horizontal plane.
• Generally
expressed an an
angle relative to
north.
• Example: N10ºE

45. Mapping Geologic Structures
– Dip (Inclination)
• The angle of inclination of the surface of a rock unit
or fault measured from a horizontal plane.
• Includes both an
inclination and a
direction toward
which the rock is
inclined.
• Example: 30ºSE

46. A Geologic Map Showing
Strike and Dip of Structures
By knowing the strike and dip, geologists can predict the nature of
rock structures hidden beneath the surface.

47. Geologist
Measuring
the Dip of
Strata in a
Roadcut