Structural Geology and Tectonics
What to know
Stress is a tensor; the principal stresses are ordered σ1 ≥ σ2 ≥ σ3. Normal faults form with vertical σ1, reverse/thrust faults with horizontal σ1, and strike-slip faults with vertical σ2. The Mohr circle, with a Coulomb failure envelope (τ = c + σn tanφ), shows how pore pressure shifts the circle left and promotes failure. Strain may be brittle (faults, joints) or ductile (folds, foliation, shear zones).
Fold terminology: anticline/syncline (younging direction), hinge line, limbs, axial surface, and plunge. Plunging folds produce V-shaped map outcrop patterns that point in the plunge direction. Faults are described by dip, hanging wall, footwall, and slip sense (normal, reverse, dextral, sinistral). Stereographic projections solve orientation problems; focal mechanisms give earthquake slip sense.
Plate tectonics provides the framework: ridges (extension), trenches and arcs (subduction), and transform faults (shear). Orogens stack thrust sheets and thicken crust, producing regional metamorphism.
Traps
Deriving fault type from map offset rather than from the stress regime and dip; confusing apparent dip with true dip; forgetting that strike-slip faults on a map show offset of markers but little topographic scarp; confusing plunge direction with dip direction; mistaking a monocline for a fault.
Rule of thumb
tan(apparent dip) = tan(true dip) × sin(angle between strike and the section line). If a section is perpendicular to strike, apparent dip = true dip.
Worked example: true dip 30° to the east, section line trending 45° from strike → tanδ' = tan30° × sin45° = 0.577 × 0.707 = 0.408, so apparent dip ≈ 22°.