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 Joint 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. © USGS http://pubs.usgs.gov/gip/earthq1/how.html

Fault Terminology Hanging Wall Footwall 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