Brittle Deformation
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Transcript of Brittle Deformation
Brittle Deformation
Remember that q is the anglebetween s 3 and a plane
Definitions
Differential Stress: Difference between largest and smallest stresses
Deviatoric Stress: Difference between total stress and meanstress.
Magnitude of Normal and
Shear Stresses
s1
s3
Normal
Shear
What does this mean for fractures?
Fractures will form when sn is low but ss is high.
q is usually 60° when fractures form
Coulomb Fracture Criterion predicts failure in rock:
ss = sn tanf
f is the angle of internal friction
tan f is the coefficient of internal friction
Rocks: Avg. is 0.6
C is the point at which sn = 0T is the point at which ss = 0
q = 90° -2f
Shaded area is stable(No brittle failure)
No failure
Failure
ExerciseSketch two cross sections:In one, s1 is horizontal, s3 vertical What is the dip of the fault predictedby this model?
In the other s3 is horizontal, s1 vertical . What is the dip of the fault?
Remember that q is the angle between s 3 and the plane. See also fig. 6.16.
Frictional SlidingOnce a fracture exists deformation continues by frictional sliding or cataclastic flow
CCW Rotation
Formation of Shear Fractures
s3
s1
2 faults form
1 fault stays active
Crack is Parallel to s1 These two are not faults.
Shallow crust
Deep crust
Non-frictional sliding on Fault surface: Calcite fibers from fluids in fault zone