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