Structural Geology and Tectonics
Structural Geology and Tectonics tests how rocks deform: strike and dip, folds, faults, joints, unconformities and plate-boundary tectonics.
Core concepts
- Strike is the compass direction of a horizontal line on a bedding plane; dip is the angle of steepest descent, measured perpendicular to strike.
- Folds: anticline (older rocks in core, limbs dip away), syncline (younger core, limbs dip toward), monocline, recumbent.
- Faults: normal (hanging wall down, extension), reverse or thrust (hanging wall up, compression), strike-slip (lateral). Joints have no displacement.
- Unconformities: angular (tilted then eroded), disconformity (erosion surface between parallel beds), nonconformity (sedimentary over igneous or metamorphic).
- Plate boundaries: divergent (ridges, normal faults), convergent (trenches, reverse faults), transform (strike-slip).
Worked example
A fault plane strikes N-S and dips 60 degrees east; the east block has moved down relative to the west block. Reasoning: the east block is the hanging wall because the plane dips east; hanging wall down means a normal fault; normal faults indicate extension. Answer: normal fault with east-side-down displacement.
Traps
Anticline versus syncline is decided by core age, not by shape alone. A reverse fault is not the same as a strike-slip fault. Dip is always perpendicular to strike; recording dip direction wrongly reverses the structure.