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Structural geology Geology 115 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 – “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 Rock fabric Terms associated with folds 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 – “foldand-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 “obliqueslip” Origin of the Transverse Ranges “Transpressional” Origin of the Sierra Nevada 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 ad Mountain Lookput Long linear ridge Digital elevation model of eastern Tennessee – note the eroded anticline and the thrust faults