Deformation

I. Deformation is a general term that refers to all changes in the original form and/or size of a rock body

A. Most crustal deformation occurs along plate margins

B. Deformation style result of strength of rock

1.brittle, resulting in broken rock,

2.or ductile, resulting in stretching, thickening, or folding

C. Factors that influence the strength of a rock

1.Temperature

2.confining pressure

3.Rock type

4.Time

II. Folds—ductile deformation

A. Rocks bent into a series of waves

B. Most folds result from compressional forces which shorten and thicken the crust

C. Types of folds

1.Anticlines and synclines

a. Anticline –arched rock layers, curved up in center

b. Syncline – bowed rock layers, curved down in center

c. Anticlines and synclines can be

1) Symmetrical - limbs are mirror images

2) Asymmetrical - limbs are not mirror images

3) Overturned - one limb is tilted beyond the vertical

d. Monocline—single limb, often over a hidden fault

e. folds can be plunging, where axis dips into Earth

2.domes and basins are regional, broad, gently dipping

a. Dome—Black Hills, South dakota

1) Circular, or slightly elongated

2) Upwarped center

3) Oldest rocks in core

b. Basin—Michigan Basin

1) Circular, or slightly elongated

2) Downwarped center

3) Youngest rocks in core

III.Faults—brittle deformation

A. Faults are fractures (breaks) in rocks along which appreciable displacement has taken place

B. Types of faults

1.Dip-slip fault

a. Movement along the inclination (dip) of fault plane

b. Parts of a dip-slip fault

1) Hanging wall – the rock above the fault surface

2) Footwall – the rock below the fault surface

c. Types of dip-slip faults

1) Normal fault

a) Hanging wall block moves down

b) Associated with fault-block mountains

c) Prevalent at spreading centers

d) Caused by tensional forces

e) Basin and Range:

i. series of horsts and grabens

ii. offset characterized by scarp

2) Reverse and thrust faults

a) Hanging wall block moves up

b) Caused by strong compressional stresses

c) Reverse fault - dips greater than 45˚

d) Thrust fault - dips less than 45˚

2.Strike-slip faults

a. Dominant displacement is horizontal and parallel to the trend, or strike

b. Transform fault

1) Large strike-slip fault that cuts through the lithosphere

2) Often associated with plate boundaries

c. San Andreas fault in California is a strike-slip fault

IV. Joints

A. Fractures along which no appreciable displacement has occurred

B. Most are formed when rocks in the outer-most crust are deformed

C. Commonly occur in intersecting sets

D. Unique styles include

1. exfoliation joints in crystalline rock

2. columnar jointing in cooled lava flows or sills