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11 -DEFORMATION OF ROCKS

11 -DEFORMATION OF ROCKS. Deformation of rocks. Folds and faults are geologic structures. Structural geology - the study of the forces that deform rocks (stress applied) and the effects of this force (strain). Small-scale Folds. Fig. 10.1. Phil Dombrowski. Small-scale Faults. Fig. 10.2.

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11 -DEFORMATION OF ROCKS

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  1. 11 -DEFORMATION OF ROCKS

  2. Deformation of rocks • Folds and faults are geologic structures. • Structural geology - the study of the forces that deform rocks (stress applied) and the effects of this force (strain).

  3. Small-scale Folds Fig. 10.1 Phil Dombrowski

  4. Small-scale Faults Fig. 10.2 Tom Bean

  5. Stress(force per unit area) Types of directed (or differential ) stresses include: • Compression - shorten • Extension - elongate • Shear - distort shape

  6. Differential stress

  7. Strength • Ability of an object to resist deformation • (low pressure vs. high pressure)

  8. Strain is any change in original shape or size of an object in response to stress acting on the object

  9. Types of deformation • Elastic • Ductile (plastic) • Brittle (rupture)

  10. Elastic deformation Temporary change in shape or size that is recovered when the deforming force is removed Think “rubber band”

  11. Ductile (plastic) deformation • Permanent change in shape or size that is not recovered when the stress is removed • Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion • Think “deck of cards”

  12. Brittle deformation (rupture) • Loss of cohesion of a body under the influence of deforming stress • Sucker breaks!!! • Usually occurs along sub-planar surfaces that separate zones of coherent material

  13. Typical stress and strain curve

  14. Factors that affect deformation • Temperature • Pressure • Strain rate • Rock type The variation of these factors determines if a rock will fault or fold.

  15. Effects of rock type on deformation Some rocks are stronger than others. competent: rocks that deform only under great stresses incompetent: rocks that deform under moderate to low stresses

  16. Effects of deformation on rock type Experimental Deformation of Marble Brittle Deformation (Under low pressure) Ductile Deformation (Under high pressure) Fig. 10.7 M.S. Patterson

  17. Orientation of deformed rocks ATTITUDE - way to describe the orientation of geologic structures. Strike: (compass) bearing of a line defined by the intersection of the plane in question and the horizontal Dip: acute angle between the plane and the horizontal, measured perpendicular to strike.

  18. Fig. 10.4

  19. Fig. 10.4

  20. Dipping Sedimentary Beds Fig. 10.3 Chris Pellant

  21. Cockscomb Ridge, S. Utah P.L. Kresan P.L. Kresan

  22. Cockscomb Ridge, S. Utah Strike Dip P.L. Kresan

  23. Tectonic Forces and Resulting Deformation Fig. 10.6

  24. BRITTLE DEFORMATION A. Abrupt movement breaks or cracks strata B. 2 kinds of breaks: • 1. JOINTS - NO movement of blocks • 2. FAULTS - YES movement of blocks 1. Movement along STRIKE 2. Movement along DIP

  25. Columns Formed by Joint-controlled Weathering Fig. 10.20 Terry Englander

  26. Joint-controlled Landscape, S.E. Utah

  27. Faults • Fractures in rocks created by • earthquakes that have moved • A. Dip-slip faults • normal • reverse • thrust • B. Strike-slip faults • right lateral or left lateral

  28. Hanging-wall and footwall

  29. Dip-slip faults Motion of the fault blocks, parallel to the dip direction.

  30. Classification of Faults hanging wall footwall cross section

  31. Normal Fault hanging wall footwall cross section

  32. Normal Dip-slip Fault

  33. Reverse Fault hanging wall footwall cross section

  34. Reverse Dip-slip Fault

  35. Drape Fold over Reverse Fault, WY George Davis

  36. Thrust Fault Thrust faults are low-angle reverse faults. hanging wall footwall cross section

  37. Thrust fault

  38. Keystone Thrust Fault, S. Nevada Cambrian Limestone Jurassic Sandstone John S..Shelton

  39. Lewis Thrust, Sawtooth Range, Wyoming Kurt N. Coonstenius

  40. French Thrust, Wyoming Mississippian Limestone Cretaceous Shale Kurt N. Coonstenius

  41. Strike-slip faults Motion of the fault blocks is parallel to the strike direction. To determine the direction of strike, put toes on the fault line & look at the direction the opposite block moved.

  42. Left-lateral Strike Slip Fault map view

  43. Right-lateral Strike Slip Fault map view

  44. Strike-slip Fault Gudmundar E. Sigvaldason

  45. Rift Valley Formed by Extension Horst Graben Horst

  46. Wildrose Graben, Southern California

  47. NASA/TSADO/Tom Stack

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