NORMAL FAULTS

Normal Fault- Extensional Dip-Slip Fault

Normal Faults: Anderson Classification

        1 is vertical, 2 and 3 are horizontal.

        3 bisects the obtuse angle

Dip-Slip Faults  

 Typically omit stratigraphic units

 HW moves down relative to FS

Displaces younger rock against older rock

Planar Faults- 

    Dip angle constant with depth

    Non-rotational faults

Listric Faults  

    Curved faults, Dips decrease with depth

    Rotational Movement of fault blocks

Synthetic Faults- 

    Dip is sub-parallel to main fault

Antithetic Faults- 

    Dip opposite to main fault

Conjugate Faults

General Occurrence of Normal Faults:

   Rift Environments

        Mid-Ocean Ridges

        Back-Arc Basins

        Continental Rifts

            Pull-Apart Basins

Formation of Rift Systems-

            Tension

             Faults: New or Reactivated

Effects of Rifting

Crustal Thinning- up to 25-50%

            Crustal Extension- up to 200-400%

             Subsidence

             High Heat Flow

             Rift Basins

Associated Structures

        Reverse Drag Folds- Roll Over Folds

       Accommodation Zones (Transfer Faults)-

       Magmatism-

                Basalt/Diabase Dikes and Sills

                Flood Basalts

                Bimodal Volcanism

       Sedimentation- Rift/Drift Sequence

                    Evaporites

            Fanglomerates

            Lacustrine

          Red Beds

          QF Clastics

Possible Causes of Rifting:

        Change in Plate Motion

         Overthickened Crust

         Convection Cells- Zones of Upwelling

         Hot Spot Plumes- Thermally activated Rifts

         Gravitational Collapse of Orogens

          Oblique Plate Motion

          Indentation and Lateral Escape

Rift-Drift Transition-              

 Transition from active to passive margin

Drifting-

  Separation of continents by sea floor spreading

    Extensional (rift) faulting ceases

   MOR-Sea floor spreading begins

Structural Geometry of Extensional Terranes

  Horst and Graben Structures-

        planar fault geometry

        Faults �die out� with depth  

  Detachment Systems-

         Listric rotational faults flatten with depth

        Sole into subhorizontal detachment

Extensional Duplexes-  

Flats and Ramps-

Antithetic and Synthetic Fault sets-

Roll Over Anticlines-

Crustal Scale Models of Extension-

        Two end members proposed

        1.Pure Shear Models-                

                Detachment at brittle-ductile transition

              Ductile deformation below detachment

         2.Simple Shear Models-

                Basal detachment 

                Soles at base of lithosphere    

                Transforms into ductile shear zone

              Explains the asymmetry of many rifts

The Fate of Rifts:

            Active Rifts

           Inactive Rifts

           Successful Rifts

           Unsuccessful Rifts- Aulacogen

Ancient Rifts:

         Keweenawan Rift   ~ 1.1byo

         Atlantic Rift          ~180-100 Ma            

         North Sea Rift       ~100-50 Ma

Metamorphic Core Complexes-

           First noted in the Basin and Range

Detachment cuts upper crustal rocks   

 Brittle rocks displaced by detachment

Isostatic uplift of Lower Crust (mylonitic)

Lower crustal (mylonitic) rocks exposed

Crustal scale, normal shear displacements;

~Simple Shear Model

Modern Rifts:

        East Africa    

        (image courtesy of USGS)           

        Basin and Range

        Lake Baikal, Russia

        Mid- Ocean Ridge Systems

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