FAULTS        

Faults

   Brittle fractures along which measurable displacement has occurred

Fault zones-

    Displacement on several brittle surfaces within a definable width.

Shear zones-

    Ductile structures along which displacement

    Occurs without mesascopic loss of cohesion;

    Involves cataclasis or crystal plastic deformation

Fault Dip Geometry and Angles:            

          Listric

                Planar                  

                         Shallow-                 <30⁰.

                        Moderate-             30⁰-60⁰.

                         Steep                      >60⁰.  

                Anastomosing

Fault block description for Non-Vertical faults:

Hanging wall block-

Foot wall block-

Dip-Slip Faults  

    Normal Faults

    Reverse Faults  

Strike-Slip Faults  

    Dextral Faults

    Sinistral Faults

Hybrid-Faults

Scissors Faults- (rotational) -

   Amount of slip changes along the strike of the fault  

Oblique-Slip Faults-

   Transtension

   Transpression  

Andersons Theory of Faulting-

   Relates principal stress axes to faulting.

   No shearing stress can act at Earths surface (air-rock interface);    

    thus, 1 of principal stress axes is parallel to the Earth�s surface.

Principal Stress Axes-   

1- bisects the acute angle between the conjugate shear fractures

2- defined by the intersection of a pair of conjugate shear fractures

  3- bisects the obtuse angle between the conjugate shear fractures.

Andersons classification of Faulting:

   Normal Faults:

        1 is vertical, 2 and 3 are horizontal.

        3 bisects the obtuse angle

     Reverse Faults:

        3 is vertical, 1 and 3 are horizontal.

        1 bisects the acute angle  

     Wrench Faults:

        2 is vertical, s1 and 3 are horizontal.

        3 bisects the obtuse angle

  Fault Displacement Terms-

    Net Slip-

   distance between 2 originally contiguous points,  offset by faulting.

        Dip slip component

        Strike slip component

Net slip vector-

   The displacement of originally adjacent points (piercing points) on opposite sides of the fault.

        Requires knowledge of:

           Magnitude of displacement (meters, feet, etc.)

           Direction of displacement (plunge and bearing)

           Sense of slip

Separation-

   Distance between displaced parts of a marker horizon,

   Measured along a specified line.

   Will vary depending upon the line chosen.

   Dip separation- offset distance measured parallel to dip of the fault

    Heave- horizontal component of dip separation

    Throw- vertical component of dip separation

Strike separation- offset measured parallel to strike of fault  

Note: slip and separation are the same only if the line along which separation is measured parallels the net slip vector.

Displacement Determination

Cutoff-

    Point on map/cross section where fault cuts pre-existing contact.

Marker Horizon-

   Distinctive feature that has been cut and offset by fault motion.

Piercing Points-

   Displacement of two originally contiguous points across a fault  

Brittle Fault Rocks-

       Fault Breccia

      Vein Filled Breccia

       Fault Gouge- rock flour

        Pseudotachylyte-

Fault Features-               

   Fault Scarps

   Surface Ruptures (Fissures)

Shear Sense Indicators-               

   Offset Markers

   Slickensides-

   En Echelon veins-

Riedel Shears-

    Fracture network associated with wrench (strike-slip) faulting. 

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