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Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
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10 Supplementary Note on Rock Mechanics Problems at Active Margins
Pages 109-112

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Select key terms on the right to highlight them within pages of the chapter.


From page 109...
... Thus, the lower crustal rocks may deform by brittle processes of fracture and frictional sliding because the stress required for this process-is lower than that for dislocation ~ creep e Qualitatively, this idea is very useful, but we lack sufficient knowledge of the constitutive parameters for likely rock types to permit useful quantitative calculations. For example, the constitutive relationship for wet quartzite is often used to represent the upper crust even though it is unlikely-that upper crustal rocks are actually wet quartzites.
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From page 110...
... This appears to explain the transition from earthquakes at shallow depths on strike-slip faults to stable sliding at greater depths, since an increase in temperature can cause a transition from velocity weakening to velocity strengthening. It might also help to explain the fact that large subduction earthquakes occur at some convergent margins and not at others, because there could be systematic differences in the rock types from one boundary to another e One possibility is that: the constitutive behavior for subducted sediments might be very dif ferent than for serpentinite or for mafic rocks.
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From page 111...
... If the normal stress and the coefficient of friction are known, the shear stress can be calculated. The existence of low angle normal faults in extensional margins, the apparently low shear stress on the-San Andreas fault, and the existence of nearly horizontal detachment faults in a number of places, such as the Los Angeles basin, are all phenomena that do not fit this simple mechanical view.
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From page 112...
... Thus, the paradox that some faults seem to move with low ratios of shear stress to normal stress is not solved by dismissing the results of laboratory experiments and known mechanical relations e There may be some reason that these concepts are not applicable to low stress faults. For instance, some other deformation mechanism might operate instead of frictional sliding If so, we need to discover what it is.
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Key Terms

  • continental margins
  • crustal rocks
  • shear stress
  • constitutive behavior
  • grain size

  • laboratory experiments
  • frictional sliding
  • fluid pressure
  • dislocation creep
  • deviatoric stress

  • pressure concept
  • diffusion creep
  • stress required
  • effective pressure
  • pressure solution

  • operative process
  • lower crustal
  • stress magnitudes
  • upper crustal
  • cataclastic flow

  • normal stress
  • physical conditions
  • sufficient knowledge
  • dominant reformational
  • operative processes

  • flow result
  • systematic differences
  • velocity weakening
  • subduction earthquakes
  • laboratory experimental


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