Faults, Folds, and Landscapes

Objectives

• Introduce basic terminology used in describing rock structure

• Distinguish between types of fault movements and the landforms they produce

• Discuss the folding of rocks and relate it to the landforms produced

• Call attention to occurrence of regional deformation of the crust, in addition to more local faults and folds

Terminology of Structure

• Strike– Line representing the

intersection of a planar feature with a horizontal plane

• Dip– Angle of a tilted bed

relative to a horizontal plane

Fault Terminology• Fault– Fracture in crustal rock

involving displacement of one side of the facture with respect to the other

• Joint– Fracture in the earth’s crust,

but with no displacement of rock bodies

An aerial view of the San Andreas fault in the Carrizo Plain, Central California. © USGShttp://pubs.usgs.gov/gip/earthq1/how.html

Fault Terminology

• Hanging Wall– Block of rocks above

the fault line

• Footwall– Block of rocks below

the fault line Figure. Image of hanging wall and footwall. © USGS NPS

Compressional Faults

• Reverse fault• Hanging wall moves up relative

to the footwall• Fault plane angle >45°

Compressional Faults

• Thrust fault• Hanging

wall moves up relative to footwall• Fault plane

angle <45°

Tensional Fault

• Normal fault– Hanging wall moves down

relative to footwall– Produced by extensional

forces

Tensional Fault

Tensional Fault Landscapes

• Basin and Range– A series of mountains

(horsts) and valleys (grabens)

• Rift Valleys– Long, narrow band of

downdropped block caused by plates being pulled apart (tension) and

Transverse Fault

• Strike-slip fault– Blocks of rock slide past

one another– Horizontal displacement

Fold Formation

• Folding– Compressional force is applied to rocks– Rocks respond by bending before fracturing

Fold Structures

• Anticlines– Archlike fold with limbs

dipping away[Insert 34.12 - anticline]

Fold Structures

• Synclines– Troughlike

fold with limbs dipping toward the axes

Anticlines

• Erosion removes upper portion of anticline• Limbs dip toward the center of fold• Older rocks are near center and becomes

progressively younger with distance

Synclines• Erosion removes

upper portion of anticline

• Limbs dip toward the center of fold

• Older rocks are near center and becomes progressively younger with distance

Plunging Folds and Associated Landscapes

• Anticlines and synclines often plunge; axes dip