Structural Geology and Plate Tectonics
What the domain covers
Structural Geology and Plate Tectonics tests how rocks deform and how large-scale plate motions produce folds, faults, earthquakes and mountain belts. Expect strike and dip, fold and fault terminology, unconformities, and evidence for continental drift and sea-floor spreading.
Core rules
- Strike is the compass direction of a horizontal line on a bedding plane; dip is the angle of steepest descent, measured perpendicular to strike.
- An anticline is an upward fold with older rocks in the core; a syncline is a downward fold with younger rocks in the core. The axial plane and fold axis describe fold geometry.
- Normal faults form under extension and have the hanging wall moved down. Reverse faults form under compression and have the hanging wall moved up. Strike-slip faults move horizontally.
- An unconformity is a break in the stratigraphic record: angular, nonconformity or disconformity.
- Plate boundaries are divergent, convergent and transform. Evidence for plate tectonics includes matching continental margins, fossils, paleomagnetism, sea-floor spreading and earthquake belts.
Worked example
Beds on both sides of a NE-SW axis dip away from the axis at about 30 degrees. The oldest beds occur along the axis. The structure is an anticline. If the beds dipped toward the axis and youngest beds were along the axis, it would be a syncline. The dip direction is perpendicular to strike, so the axis orientation controls the map pattern.
Traps
Normal faults are not caused by compression; they record extension. Do not confuse anticline and syncline by shape alone; use rock age and dip direction. Strike-slip faults do not necessarily produce a scarp with large vertical displacement.