BRITTLE STRUCTURES

Brittle Deformation-

        -Permanent deformation of a solid

        -Characterized by development of fractures.

Brittle Structures include:

    Fracture- discontinuity surface.

     Vein- fracture in which minerals have precipitated from solution.

     Dike- fracture filled by a rock intrusion

                Intrusion may be igneous (more common) or sedimentary

Steeply dipping Dike in the Anti-Atlas Mountains of Morocco

Joint- fracture with no measurable shear displacement

Veins and Faults in Siroua, Morocco

Shear fracture-  limited amounts of shear displacement

Fault- fracture surface with measurable displacement

Conjugate faults in Migmatite, Siroua, Morocco

Shear Zones- mixed brittle/ductile deformation 

            Cataclasis, crystal plastic deformation and diffusion.

    Cataclasis- brittle deformation mechanism

           Macroscopic ductile flow

            Grain-scale fracturing, crushing, and frictional sliding

Shear zone within Migmatite, Siroua, Morocco

Modes of Fracture Development:

   3 basic fracture modes representing �end members�

   can describe any combination of joint and shear fracture

Mode I:   Tensile cracks-

Fractures open slightly in direction perpendicular to crack surface

Mode II:   Shear Fracture              

                    Shear by horizontal sliding- rocks on one side of crack surface move slightly parallel to fracture surface

Mode III: Shear by scissor motion; Tearing mode

Mode I Tensile Fractures

Problem:

Theoretical rock strength is ~ 500-5000 MPA

Actual rock strength                ~ 10MPA

Solution:

Griffith Cracks-

concept proposed by A.W. Griffith in the 1920�s.

Proposed that pre-existing micro-cracks and flaws in the rock, including grain-scale fractures, pores and grain boundaries, induce fracturing at anomalously low stresses.

i.e., mesoscopic cracks grow from pre-existing flaws (Griffith cracks)  in the rock

 Griffith Cracks suggest:

1.Total length of the crack does not form instantaneously;

2.Rather, it initiates at a small flaw and then grows outward.  

3.Stress is concentrated at flaw zones within rocks.

              Analogies: tear in paper, windshield crack, ice

Tensile Fractures (Mode I) may occur under 3 conditions:

1.Tensile stress- Axial Stretching

2.Axial Compression with low confining pressure

3.Hydraulic Fracturing- increasing pore pressure.

Brittle Faulting-produced by two ways:

                Rupture of previously intact rock

                Reactivation of weak surfaces (joints, foliations, faults)

Brittle Faulting Processes:

                Frictional sliding

                Slip by growth of fault parallel veins-

                        presence of water and slow rate of movement

                Cataclastic flow- microcracking and frictional sliding, 

                       rotation and transport of fragments.

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