Physical Geology Chapter 15 Faults

 

Forces on rocks…and what happens

Plate tectonics can generate tremendous

   forces that can bend, fold & break rocks

 

Stress and strain

Stress is the force applied to a rock

 

Strain is the resulting deformation of the rock

 

Types of stress and strain:

Figure 15.2  compression and tension on silly putty

Pushing – compressional

Pulling – extensional

Shear stress Force from two opposing directions

Figure 15.3

 

The elastic limit – Rocks may stretch, but only so far

Figure 15.4

Elastic – rock deforms, but can return to its original shape  (rubber band)

Ductile (or plastic) – rock deforms, but does not return

          to its original shape  (silly putty or toothpaste)

Brittle – past the elastic limit, rock breaks 

                   (rubber band snaps)

 

Words to explain angles of rocks

Principle of Original Horizontality

          most sedimentary rocks were deposited in flat-lying layers

 

  examples:  lake bed (ancestral Lake Erie) 

                    continental shelf

Figure 15.7  strike and dip

 

Dip – a marble would roll down this direction

                  

(and apparent dip)

 

Strike – intersection of the dipping bed and a horizontal plane

                   (think of water level)

 

 

 

Types of faults, directions of motion  Figure 15.21

 

Dip-slip faults:  Vertical movement along the fault plane

           

Reverse fault

         

 

 

 

 

 

Normal fault

Strike-slip fault   Lateral movement along the fault plane

 

Oblique-slip fault

Types of strike-slip faults

 

 

 

right-lateral – if you are standing, facing the fault,

the other block is moving to your right

 

 

left-lateral – movement to the left

 

 

Paired faults form structures:

Figure 15.23          Graben (“ditch”)    Horst

Figure 15.24  Photo of a graben

 

Normal faults

 

Low-angle thrust fault  Figures 15.25, 15.26

 

What might be a difference between

  low-angle and high-angle thrust faults?

 

 

Large-scale thrust faults

 

Chief Mountain thrust fault in Glacier National Park, Montana

Precambrian (older) rocks over Cretaceous (younger)

 

Clues for finding faults

 

 

Lineations    Offsets of streams    Long lakes

 

Figure 15.28  features associated with strike-slip fault

 

Offset bedding

 

Stream offset

  Box 15.3, Figure 3

Fault scarp and rills

 

Sag pond (in fault zone)

 

Offsets of geologic features

 

    Example: Cindercone in Nevada

 

San Andreas fault zone

A whole system of faults