how rocks deform brittle-ductile behavior faulting and folding deformation of rocks
TRANSCRIPT
How Rocks DeformBrittle-Ductile BehaviorFaulting and Folding
Deformation of Rocks
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
Strain
Stress
Elastic behavior
Ductile behavior
X
Fracture, breaks
Permanent strain
Rock
Rubber band
Relationship between stress and strain
Strain
Stress
Brittle behavior:Very little ductile deformation before fracturing
X
Fracture
X
Ductile behavior:Extensive ductile deformation before fracturing
Ductile Brittle
Ductile BehaviorFolding of Rocks
Brittle BehaviorFaulting 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 stressElastic limit
Effects of Temperature and Strain Rate
Brittle-Ductile Transition
Mantle
Crust
surface
Low TemperatureLow Pressure 15-20 km
Brittle
DuctileHigher TemperatureHigher Pressure
Limits the depths of earthquakes
schematic strength profile
through continental lithosphere
StrainS
tress Yield
strength=0
T=1300 C
Lithosphere-Asthenosphere
Deformation in Progress
Abrupt Movement along Faults
Uplifted sea floor at Cape Cleare, Montague Island, Prince William Sound. Uplift about 33 ft
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.
upliftsubsidence
LA
SA
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 relativerelative 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 faultDirection of relative movement
Normal Fault (dip-slip)
Normal Faulting
Hanging wall
Foot wall
Tetons – fault range scale
Basin and Range
Normal FaultingHorst-Graben Structures
Death Valley, CA
Reverse Fault (dip slip)
> 45° dip
Reverse Faults
Thrust Fault (dip-slip)
< 45° dip
Older rocks
Younger rocks
Thrust Fault
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-lateralStrike-slipStress: shear
Types of FoldsDuring 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