Brittle Deformation Remember that  is the angle between  3 and a plane

Definitions Differential Stress: Difference between largest and smallest stresses Deviatoric Stress: Difference between total stress and mean stress.

Magnitude of Normal and Shear Stresses 11 33 Normal Shear

What does this mean for fractures? Fractures will form when  n is low but  s is high.  is usually 60° when fractures form

Coulomb Fracture Criterion predicts failure in rock:  s =  n tan   is the angle of internal friction tan  is the coefficient of internal friction Rocks: Avg. is 0.6 C is the point at which  n = 0 T is the point at which  s = 0  = 90° -2 

Shaded area is stable (No brittle failure) No failure Failure

Exercise Sketch two cross sections: In one,  1 is horizontal,  3 vertical What is the dip of the fault predicted by this model? In the other  3 is horizontal,  1 vertical. What is the dip of the fault? Remember that  is the angle between  3 and the plane. See also fig

Once a fracture exists deformation continues by frictional sliding or cataclastic flow

CCW Rotation

Formation of Shear Fractures 33 11 2 faults form 1 fault stays active

Crack is Parallel to  1 These two are not faults.

Shallow crust Deep crust

Non-frictional sliding on Fault surface: Calcite fibers from fluids in fault zone