TECTONICS - Introduction to Faults and Folds

 

Tectonics & Mountain building

Associated with plate tectonics

Most common at Plate boundaries

Two (or more) plates interact along huge linear zones of faulting

Tectonic activity is common here

The amount of energy involved here is immense

 

Crustal stress

The movement of large areas of the crust vertically & horizontally

Immense stresses at an extremely slow rate (2nd Law of GeoFantasy)

Different sections of the crust are moving at different velocities

Therefore they interact at their edges - plate margins

All this up and down involves distorting the crust

Several things can happen

Break - fractures & joints

Break & slip - faults

Fold

What happens depends on

Rock type

Temperature

Type and magnitude of the force (stress)

 

Breakage in the crust

When cold rock moves, it can break

Must be relatively cold and brittle

Called fractures, or joints

Breakage without movement

Faults: A fracture where the sides have moved relative to each other

Usually a planar surface

Hangingwall vs. footwall

Several types of faults, based on the relative sense of motion

Normal faults - hanging wall down

Usually the result of tension and crustal lengthening

Horst & Graben (see fig. 8-13; pg. 171)

Actually Valley Building, not Mountain Building

Reverse Fault - hanging wall up

Common at convergent boundaries

Results in shortening of the crust

Strike-slip faults

Offset spreading centers

 

Folding in the crust

Folding as a result of directed stress at depth

Below the Brittle-Ductile Transition Zone

Generally associated with convergence & shortening of crust

Push a rug against the wall

Anticline/syncline

Come in sets - rarely alone