Folds, Faults and other manifestations of rock deformation

Rocks can be folded or faulted. Evidence of great forces and movement within the crust. Tilting of rocks is least we can obsreve.

Forces deforming rocks

Compressive

Tensional

Shearing

Controls on breaking or bending- brittlelness and ductility

Rock breaks - brittle. Rock bends, or otherwise acts like modeling clay - ductile

Type of deformation	Temperatures	Rate of deformation
Brittle	Cold (ie <300 degreesC)	rapid
Ductile	hot (>300 degrees C)	slow

All materials like this - metals, chocolate, rocks

Therefore, deeper in earths crust (>10-15 km) rocks are generally ductile

Brittle structures

o Fractures and joints

Crack with no displacement.

Barren vs mineralized

o Faults

Fracture with subsequent displacement

Extension - results in Normal faults ; horsts and grabens

Compression - results in thrust faults

Compression - can also result in strike-slip faults

Ductile structures

o Fabrics - foliations and schistosity

o Folds all layered materials wil wrinkle of fold when shortened parallel to layering

Fold anatomy:

Limbs, axis (hinge), axial plane

syncline/anticline

symmetrical/asymmetrical

plunging folds; domal structures (doubly plunging) - oil reservoirs

Mapping Geological Structures

Field data, maps and sections

In order look at deformation much of data has to be taken in the field. Folded rocks only partly crop out and we may see only tilted limbs.

Strike and dip of beds

Strike measured relative to N - with compass

dip with respect to horizontal - clinometer

Can be applied to all planar features

Plotting on a map

Cross sections

Mountain Belts
zones of high deformation in the crust

Fold and thrust belts - great deformation in the past due to continental collision

Active deformation - neotectonics

Most of deforemation we see in mountain belts took place in the disytant past and rock is no longer deforming.

At active plate boundaries, however, there may be active deformation. Earthquakes

When have this may disturb the Earths surface. So called "fault lines" Can see structures because not yet eroded

Example: - San Andreas fault zone - strike slip

Rift valleys: extension inthe crust eg. East African Rift; Rhine graben - actually forms a valley