lec11_structures_folds_faults

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Transcript lec11_structures_folds_faults

Deformation of Rocks
How Rocks Deform
Brittle-Ductile Behavior
Faulting and Folding
Stress and Strain
• The keys to understanding any
deformation are stress (the cause) and
strain (the effect)
Compression
• Rocks are squeezed
or compressed by
forces directed
toward one another.
• Rocks are
shortened by
folding or faulting
Plate Boundary: Convergence Zones
Tension
• Rocks are
lengthened or pulled
apart by forces
acting in opposite
directions
• Rocks are stretched
and thinned
Plate Boundary: Divergence Zones
Shear
• Forces act parallel
to one another but
in opposite
directions
• Results in
displacement of
adjacent layers
along closely spaced
planes
Plate Boundary: Transform Faults
Relationship between stress and
strain
Elastic behavior
X
Rock
Stress
Fracture, breaks
Ductile behavior
Rubber band
Permanent strain Strain 
Relationship between stress and
strain
Brittle behavior:
X
Very little ductile
deformation before
fracturing
X
Fracture
Stress
Strain 
Ductile behavior:
Extensive ductile
deformation before
fracturing
Ductile
Brittle
Ductile Behavior
Folding of Rocks
Brittle Behavior
Faulting of Rocks
What controls brittle vs. ductile?
–
–
–
–
Rate of deformation (fast = brittle)
Rock strength (strong = brittle)
Temperature (cold = brittle)
Confining pressure (shallow = brittle)
• Just remember deeper = ductile
– Near surface= rocks are brittle
– At depth= rocks are ductile
What controls brittle vs. ductile?
Rate of deformation (strain rate)
Low strain rates Ductile (Mantle
Convection)
High strain rates  Brittle (Earthquake
waves)
Yield stress
Elastic limit
Effects of Temperature and Strain Rate
Brittle-Ductile Transition
Limits the depths of
earthquakes
surface
Low Temperature
Low Pressure
Crust
Mantle
Higher Temperature
Higher Pressure
Brittle
15-20 km
Ductile
schematic
strength
profile through
continental
lithosphere
Stress
T=1300 C
Lithosphere-Asthenosphere
Yield
strength=0
Strain
Deformation in Progress
Abrupt Movement along Faults
Uplifted sea floor at Cape Cleare, Montague Island, Prince
William Sound. Uplift about 33 ft
LA
SA
uplift
subsidence
Gradual Movement: Perspective view of the Los Angeles region with
superimposed InSAR( Interferometric Synthetic Aperture Radar)
measurements of ground motions between May and September 1999. Large
regions of metropolitan Los Angeles are rising and falling by up to 11 cm
annually, and a large portion of the city of Santa Ana is sinking at a rate of
12 mm per year.
Past Deformation: Folding
Large scale and small
scale folds
Folding: large and
small scale
Past Deformation: Faulting
Large scale and
small scale
Strike and Dip
Measuring Deformation
in the Rocks
Strike & Dip
Faults
• Fractures along which there is
relative motion parallel to the
fracture
• The fracture is called the fault plane
– Vertical motion (dip-slip)
– horizontal (strike-slip).
– Most faults have a combination of both
types of motion (oblique).
Types of Faults
Classified according to:
Dip of fault
Direction of relative movement
Normal Fault (dip-slip)
Normal Faulting
Foot wall
Hanging wall
Tetons – fault range scale
Basin and
Range
Death Valley, CA
Normal Faulting
Horst-Graben
Structures
Reverse Fault (dip slip)
> 45° dip
Reverse Faults
Thrust Fault (dip-slip)
< 45° dip
Thrust Fault
Older rocks
Younger rocks
Thrust Faults. Snake Range, Wy
Strike-Slip Fault (horizontal motion, no vertical motion)
Strike-Slip Fault
San Andreas
Fault
•
Transform plate
boundary
(Pac / N.A.)
•
System of right
lateral faults
Offset Streams (San Andreas Fault)
A pair of streams that has been offset by right-lateral slip on the San Andreas
fault (lineament extending from left to right edge of photograph). View
northeastward across fault toward the Temblor Range. Photograph by Sandra
Schultz Burford, U.S. Geological Survey.
Strike-slip fault
Off-set stream
Right-lateral
Strike-slip
Stress: shear
Types of Folds
During mountain building or compressional
stress, rocks undergo ductile deformation to
produce folds
anticline
syncline
Types of Folds
Anticline: Warped upwards. Limbs dip
outward. When eroded, oldest rocks
crop out in the center (assuming
everything is right-side-up).
Syncline: Warped downwards. Limbs
dip inward. When eroded, youngest
rocks crop out in the center (assuming
everything is right-side-up).
Basins and Domes resemble anticlines & synclines
 vertical motions instead of lateral motions
Stress, Strain & Plate
Tectonics
• Plate collisions (convergent margins)
– Compressive strsses
– Folds & reverse faults
Stress, Strain & Plate
Tectonics
• Divergent plate boundaries
– Tensional stresses
– Normal faults
Stress, Strain & Plate
Tectonics
• Transform plate boundaries
– Shear stress
– Transform faults