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).
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.
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.
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).
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.
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.
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
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.
