Geology 115

• 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 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

• 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)

• Normal faults arise from extensional stress
• Called “ normal ” because of age relationship of rocks across the fault
• Detachment faults are low-angle normal faults

“ Classic ” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “ foldand-thrust ” belts

“ Classic ” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “ foldand-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

• 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 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

• 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 °

“ Classic ” plate tectonic settings and mountain building
• Divergent boundary – fault-block mountains (horsts and grabens)
• Collision (convergent) boundary – “ foldand-thrust ” belts
• Subduction (convergent) boundary – volcanic arc mountains
• Transform boundaries are not associated with mountain-building

• Strike-slip faults result from shearing stress
• Called “ strike-slip ” because motion is along strike (horizontal orientation)
• Types: left-lateral, right-lateral

• Mountain building occurs in strike-slip fault areas with some compression
• Called “ obliqueslip ”

“ Transpressional ”

Relict subduction zone

Gettysburg NMP
(1895) in 1863

All of the folding in the
Gettysburg area is due to the collision of African and
North American plates and the closing of the Iapetus
Ocean 325 – 280 million years ago

“Thin-skinned” tectonics = fold-and-thrust belt = a region where there has been a shortening of the crust due to tectonic compression , and a series of folds and thrust faults have been created, with the thrust faults all connecting at a relatively shallow depth at a décollement

Gettysburg basin, and other basins aligned with it, developed as a result of the rifting of the supercontinent
Pangaea and the opening of the
Atlantic Ocean about
220 mya

Chickamauga and Chattanooga NMP (1890)

Moccasin Bend
Railro
Long linear ridges

Digital elevation model of eastern Tennessee – note the eroded anticline and the thrust faults