Structural Geology and Tectonics
Domain focus
Structural Geology and Tectonics tests stress and strain, fault and fold geometry, strike/dip reasoning, plate kinematics, and orogenic systems. Questions often give orientation data and ask for slip sense, fold axis, or shortening direction.
Core rules and concepts
- Strike is the compass direction of a horizontal line on a plane; dip is the steepest angle perpendicular to strike. Apparent dip is always less than true dip unless the section is perpendicular to strike.
- Anderson's fault types: normal faults form under extension (σ1 vertical), reverse/thrust under compression (σ1 horizontal), strike-slip under horizontal shear (σ1 and σ3 horizontal, σ2 vertical).
- In a fault, the hanging wall is above the fault plane. Normal faults drop the hanging wall; reverse faults raise it. Slickensides and drag folds help determine slip.
- Fold axis is the line of maximum curvature; axial plane bisects the limbs. A vertical axial plane with horizontal axis gives a symmetric upright fold.
Worked example
Two limbs of a fold strike N–S and dip 40° W and 40° E. The fold axis is the intersection of the two limb planes. Because both planes strike N–S, their intersection is a horizontal line trending N–S. The axial plane is vertical and also strikes N–S. This is an upright fold with a horizontal N–S axis; the reasoning uses the geometric intersection, not the dip directions alone.
Traps
Do not call a steep fault reverse without checking slip sense. Do not report apparent dip as true dip. Do not confuse a fold axis with an axial trace on a map. In stereonet problems, check whether you are plotting planes or poles; the two answers differ by 90°.