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The Mechanics of the crust

The Mechanics of the crust. How do rocks deform in the crust ? Mechanisms Bearing strength Must consider: Brittle crust Ductile crust. The Brittle and the Ductile regime. Brittle. Ductile. Brittle : Fault Breccia.

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The Mechanics of the crust

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  1. The Mechanics of the crust How do rocks deform in the crust ? • Mechanisms • Bearing strength Must consider: Brittle crust Ductile crust

  2. The Brittle and the Ductile regime Brittle Ductile

  3. Brittle :Fault Breccia

  4. cataclastic intermediate betweenbrittle and ductileFault Gouge =>

  5. Ductile : Mylonite

  6. Triaxial experiment allow to impose various confining (blue) and deviatoric (red – blue) normal stresses

  7. The regime stress-strain curves evolveswhen the confining pressure increases

  8. The effect of increasing confining pressure Confining stress

  9. Two distinct types of plasticity Strain Hardening Plasticity At depth : Strain Hardening Plasticity : can accommodate permanent strain without losing the ability to resist load At The surface : Strain Softening Plasticity: its ability to resist load decreases with permanent strain Strain Softening Plasticity

  10. The mode of failure evolves when increasing the confining pressure Cataclastic Brittle

  11. IMPORTANT The loss of resistance of the upper crust While the lower crust is still resistant Is responsible for the the earthquakes instability : For a given applied force the displacement should be infinite Infinite displacement in the upper crust upper crust force

  12. The Brittle regime Responsible for the faults close to the surface

  13. Common Observation :Conjugate Shear Fractures Conjugate fractures Are pair of fault which Slip at the same time They have opposite shear senses

  14. On rock experimentsconjugates fault are also observed inthe lab.

  15. Mechanical Explanation of conjugates faults

  16. Byerlee’s Law rule for rock friction deduced from triaxial experiment(t-s : mohr space) t NOT STABLE STABLE s

  17. Impossible Stress State • Any stress state whose circle lies outside the envelope is an unstable stress state, and is not physically possible • Before stress reaches this state, the sample would have failed

  18. Failure Envelope The Shaded stable Area is bounded by the failure envelope in black

  19. Stable Stress State Any stress state lying within the envelope is stable

  20. Defining Stress State Stress state tangent to the envelope defines the failure state A fault forms…

  21. Coulomb Criterion The failure enveloppe is linear The further away from the origin the circle center is, the larger is the radius of the circle The bigger is the maximum compression

  22. Coulomb’s Criterion • t = σs= C + μ σn • C is a constant that specifies the shear stress necessary to cause failure if the normal stress is zero (order 10 Mpa) • The two fractures occur at an angle fº, and correspond to the tangency points of the circle representing the stress state at failure with the Coulomb failure envelope

  23. Byerlee’s Law rule for rock friction deduced from triaxial experiment(t-s : mohr space) • For σn < 200 MPa, • For 200MPa < σn < 5000MPa where: t = shear stress (MPa) sn = normal stress (MPa)

  24. Possibles Applications : Thrust are usualy diping under 30° Continental deformation

  25. The dip angle can serve to define the friction associated with the earthquake Exercice what is the friction angle Here subduction zone

  26. Exercice : Conjugates faults Plot on a stereonet the conjugates faults Fa) Strike : 25°E, dip : 35°E Fb) Strike : 30°W, dip : 15°W Measure the angle between the fault planes Deduce the internal friction angle and the principal directions of compression and extension

  27. Normal Faults close to the surface Here the two normal Faults are conjugate Faults they typically form an angle of 60°

  28. Conjugates Fault and Stress • Conjugate shear fractures develop at about  = 30 degrees from the maximum compressional stress : 1 • 1 bisects the acute angle of about 60o between the two fractures • The minimum compressional stress 3 bisects the obtuse angle between the two fractures

  29. Conjugates Faults and the Principal Stresses • Reverse faults are more likely to form if 3 is vertical and constant (at a standard state), while horizontal, compressive 1 and2 increase in value compared to the standard state • Normal faults form if 1 is vertical and constant, while horizontal 3 and2 decrease in value, or if horizontal 3 is tensile • Strike-slip faults form if 2 is vertical and constant, while horizontal 1 and2 increase and decreases in value, respectively NEAR the SURFACE

  30. Brace-Goetze strength profiles Brittle Ductile After Kohlstedt et al., 1995

  31. Frictional Sliding • Frictional force does not depend on the shape of the object • Both objects, of the same mass, have the same sliding force, despite having different areas of contact

  32. Amonton’s Law • Frictional resistance to sliding  normal stress component across the surface • First “published” account this empirical law of friction was made by the French physicist Guillaume Amonton in 1699, although Leonardo da Vinci’s notes indicate he knew of the result about 200 years earlier • If normal stress increases, the asperities are pushed more deeply into the opposing surface, and increasing resistance to sliding

  33. Fracture Surface • Fracture surface, showing voids and asperities (Figure 6.23a, text) • As another, also bumpy, surface tries to slide over the first surface, their asperities interact, causing friction

  34. Real Area of Contact • The bumps mean that only a small part of the surfaces are actually in contact • Dark areas are real area of contact (RAC) (Figure 6.23c, text)

  35. Surface Anchors • The forces normal to these surfaces will be concentrated on the small areas in contact • Asperities cumulatively act as small anchors, retarding any slippage along the surface (Figure 6.23b, text)

  36. Criteria for Frictional Sliding • Before the initiation of frictional sliding, enough shear must be present to overcome friction • We can define a criterion for frictional sliding to represent the necessary shear • Experimental work has shown that, independent of rock type, the following criterion holds • σs/σn = constant

  37. Movement of Stress Along σ3 Axis • When represented on a Mohr diagram, the Mohr circle moves to the left along the normal stress axis • Figure 6.27 in text

  38. Pore Pressure and Shear Fracturing • Pore pressure also plays a role on shear fracturing • Since pore pressure counteracts the confining pressure, we can rewrite the equation for shear stress to take pore pressure into account: • σs = c + μ(σn - Pf)

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