Structural geology

Geology 101, Fall 2012

Structural geology

• The study of the deformation and fabric of rocks in order to understand the tectonic forces

• Rheology is the study of the effect of stress on materials

Stress-strain diagram

• Stress is measured as a force applied to a material

• Strain is the resulting change in volume of the material

• Elastic means that the material returns to its normal volume once the stress is removed; plastic (or ductile) means that it does not

Earth’s interior structure

• In fact, using seismic studies, we can measure the depth at which the Earth’s materials reach the elastic/ductile point

• This is where the material becomes “gooey”

The stress-strain diagram is misleading

• It shows only one type of tectonic stress -- compression

• Two other types -- extension and shearing

“Classic” plate tectonic settings and mountain building

• Divergent boundary – fault-block mountains (horsts and grabens)

Extensional stress

• Normal faults arise from extensional stress

• Called “normal” because of age relationship of rocks across the fault

• Detachment faults are low-angle normal faults

Orogeny - horst and graben

“Classic” plate tectonic settings and mountain building

• Divergent boundary – fault-block mountains (horsts and grabens)

• Collision (convergent) boundary – “fold-and-thrust” belts

“Classic” plate tectonic settings and mountain building

• Divergent boundary – fault-block mountains (horsts and grabens)

• Collision (convergent) boundary – “fold-and-thrust” belts

• Subduction (convergent) boundary – volcanic arc mountains

Compression leads to certain structures

• Specifically, ductile structures called folds

• Sedimentary rocks can be deformed this way, but some metamorphism may also occur

Rock fabric

Terms associated with folds

How folds look on the surface

• The combination of folding and differential erosion creates interesting rock outcrop patterns, which help explain human history.

Plunging folds• Because the whole

fold may be tilted perpendicular to the axial plane, folds may plunge

• The plunge is measured as an orientation and an angle off of horizontal

But even the toughest rocks break, and the break is called a fault

Faults• Faults are a break in

a rock along which offset has clearly occurred

• Breaks where there is no evidence of motion are called joints

• All types of tectonic stress may lead to faults

Compressive stress

• Reverse faults result from compression

• Called “reverse” because of age relationship of rocks across the fault

• Thrust faults are reverse faults with a fault dip angle < 45°

Orogeny - fold and thrust belt

“Classic” plate tectonic settings and mountain building

• Divergent boundary – fault-block mountains (horsts and grabens)

• Collision (convergent) boundary – “fold-and-thrust” belts

• Subduction (convergent) boundary – volcanic arc mountains

• Transform boundaries are not associated with mountain-building

Shearing stress

• Strike-slip faults result from shearing stress• Called “strike-slip” because motion is along

strike (horizontal orientation)• Types: left-lateral, right-lateral

Orogeny - transpressional

• Mountain building occurs in strike-slip fault areas with some compression

• Called “oblique-slip”

Origin of the Transverse Ranges

“Transpressional”

Origin of the Sierra Nevada

Relict subduction zone