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Deformation of Rocks

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.

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Deformation of Rocks

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  1. Deformation of Rocks How Rocks Deform Brittle-Ductile Behavior Faulting and Folding

  2. Stress and Strain • The keys to understanding any deformation are stress (the cause) and strain (the effect)

  3. Compression • Rocks are squeezed or compressed by forces directed toward one another. • Rocks are shortened by folding or faulting

  4. Plate Boundary: Convergence Zones

  5. Tension • Rocks are lengthened or pulled apart by forces acting in opposite directions • Rocks are stretched and thinned

  6. Plate Boundary: Divergence Zones

  7. Shear • Forces act parallel to one another but in opposite directions • Results in displacement of adjacent layers along closely spaced planes

  8. Plate Boundary: Transform Faults

  9. Rock Stress Rubber band Strain  Relationship between stress and strain Elastic behavior Fracture, breaks X Ductile behavior Permanent strain

  10. 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

  11. Ductile Brittle

  12. Ductile Behavior Folding of Rocks Brittle Behavior Faulting of Rocks

  13. 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

  14. What controls brittle vs. ductile? Rate of deformation (strain rate) Low strain rates Ductile (Mantle Convection) High strain rates  Brittle (Earthquake waves)

  15. Yield stress Elastic limit Effects of Temperature and Strain Rate

  16. Brittle-DuctileTransition Limits the depths of earthquakes surface Brittle Low Temperature Low Pressure 15-20 km Higher Temperature Higher Pressure Ductile Crust Mantle

  17. T=1300 C Yield strength=0 Stress Strain Lithosphere-Asthenosphere schematic strength profile through continental lithosphere

  18. Deformation in Progress

  19. Abrupt Movement along Faults

  20. Uplifted sea floor at Cape Cleare, Montague Island, Prince William Sound. Uplift about 33 ft

  21. 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.

  22. Past Deformation: Folding Large scale and small scale folds

  23. Folding: large and small scale

  24. Past Deformation: Faulting Large scale and small scale

  25. Strike and Dip

  26. Measuring Deformation in the Rocks Strike & Dip

  27. 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).

  28. Types of Faults Classified according to: Dip of fault Direction of relative movement

  29. Normal Fault (dip-slip)

  30. Normal Faulting Foot wall Hanging wall

  31. Tetons – fault range scale

  32. Basin and Range Death Valley, CA Normal Faulting Horst-Graben Structures

  33. Reverse Fault (dip slip) > 45° dip

  34. Reverse Faults

  35. Thrust Fault (dip-slip) < 45° dip

  36. Thrust Fault Older rocks Younger rocks

  37. Thrust Faults. Snake Range, Wy

  38. Strike-Slip Fault (horizontal motion, no vertical motion)

  39. Strike-Slip Fault

  40. San Andreas Fault • Transform plate boundary (Pac / N.A.) • System of right lateral faults

  41. 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.

  42. Strike-slip fault Off-set stream Right-lateral Strike-slip Stress: shear

  43. anticline syncline Typesof Folds During mountain building or compressional stress, rocks undergo ductile deformation to produce folds

  44. Types of Folds

  45. Anticline: Warped upwards. Limbs dip outward. When eroded, oldest rocks crop out in the center (assuming everything is right-side-up).

  46. Syncline: Warped downwards. Limbs dip inward. When eroded, youngest rocks crop out in the center (assuming everything is right-side-up).

  47. Basins and Domes resemble anticlines & synclines  vertical motions instead of lateral motions

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