Rock deformation

Geology 1001-section 4

Dr. Dutrow

last updated: 02/28/98

ROCK DEFORMATION (abbreviated versions of the overheads)

Earth processes are dynamic - internal changes must involve intense deformation.

A record of this deformation is found in the geological structures; especially in folds

and faults

Geologic mapping - geologists make maps based on the distribution of outcrops that include features such as the geographic distribution of outcrops, the type of rocks in the outcrops and the attitude of the beds or layers as measured by the strike (direction of a rock layer relative to the horizontal) and dip (the angle the layer is inclined relative to the horizontal).

the geologic map forms the basis for reconstructing the geologic history in a 3-D puzzle

interpretative cross-sections are commonly employed

ROCK BEHAVIOR

Experiments on deformation of rocks show there are several factors that influence rock deformation.

rock
presence or absence of
nature and intensity of
time of

Ductile deformation = (the smooth, continuous, "plastic" deformation) takes place at .

Brittle = deformation (sudden breakage) takes place at

3 types of forces affect rock deformation

(1) compressional
(2) tensional
(3) shearing

FOLDING

Folding implies a planar feature that has been bent.

these are typical of a mountain range and can develop at the cm or km scale
may be gently warped or severely bent
folding is commonly the only evidence of a former mountain range

To characterize and interpret a fold, a geologist = typically looks at the surface information e.g. dip, strike, "younging" information, = etc.

Anatomy of a fold

(1) Upfolds =3D anticlines (oldest rocks in the )
(2) Downfolds =3D synclines (youngest rocks in the )

Parts of a fold

(1) limbs
(2) axial plane
(3) axis

Types of folds

FRACTURES AND FAULTS

Joints - cracks in a rock with

a regularity in the joint system implies a regularity

joints reflect the ancient

Faults - a fracture in a rock with

the type of fault is established by the direction of motion

(1) dip-slip - up/down motion in

(a) normal = fault - rocks above the fault plane move

(b) reverse fault - rocks above the fault plane move

(2) strike-slip (right or left lateral) -

(3) oblique-slip - combination of

Grabens (rift valley) - narrow valley bounded by one or more

Topographic expression of deformation - deformation over the past few 10s of Ma produce the relief we see in most mountain ranges

we can infer deformational processes (and tectonic processes) from preserved deformational features.