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
Tension • Rocks are lengthened or pulled apart by forces acting in opposite directions • Rocks are stretched and thinned
Shear • Forces act parallel to one another but in opposite directions • Results in displacement of adjacent layers along closely spaced planes
Rock Stress Rubber band Strain Relationship between stress and strain Elastic behavior Fracture, breaks X Ductile behavior Permanent strain
Stress Strain Relationship between stress and strain Brittle behavior: Very little ductile deformation before fracturing X X Fracture Ductile behavior: Extensive ductile deformation before fracturing
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-DuctileTransition Limits the depths of earthquakes surface Brittle Low Temperature Low Pressure 15-20 km Higher Temperature Higher Pressure Ductile Crust Mantle
T=1300 C Yield strength=0 Stress Strain Lithosphere-Asthenosphere schematic strength profile through continental lithosphere
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
Past Deformation: Faulting Large scale and small scale
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 Faulting Foot wall Hanging wall
Basin and Range Death Valley, CA Normal Faulting Horst-Graben Structures
Reverse Fault (dip slip) > 45° dip
Thrust Fault (dip-slip) < 45° dip
Thrust Fault Older rocks Younger rocks
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
anticline syncline Typesof Folds During mountain building or compressional stress, rocks undergo ductile deformation to produce 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