True/False Statements: 18-Geol-A4 Structural Geology, December 2017, Q1

Question A: True/False Statements (1 mark correct, −0.5 incorrect, blanks=0 – 20 marks total)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Question A — True/False answers, all 20 statements
#	Answer	Justification
1	False	Thrust faults propagate the opposite way on both counts: they break up-section (cutting progressively higher stratigraphic levels in the transport direction) and propagate toward the foreland (away from the hinterland) in the classic in-sequence, piggyback style.
2	True	By definition an upright fold has a near-vertical (steeply dipping) axial surface; on the limbs, bedding dips at some lesser angle away from the hinge, so the axial surface is steeper than the bedding it bisects.
3	False	The relationship is reversed: folding is the ductile/plastic process (continuous, cohesive deformation), while fracturing is the brittle process (discrete loss of cohesion).
4	False	The axial surface (a plane, strike/dip) fixes only where the hinge lies within that surface, not the trend/plunge of the hinge line itself — two folds can share the same axial surface but have different hinge-line plunges (e.g. non-cylindrical or doubly-plunging folds), so BOTH the axial surface and the fold axis are needed to completely describe orientation.
5	False	A stress state plotting outside (above) the failure envelope is not physically sustainable — the rock ruptures before that state is reached. Stability corresponds to points on or below (inside) the envelope, the opposite of what the statement claims.
6	False	Mode I (extension) cracks open perpendicular to σ₃ (the least principal stress), not σ₁ — the crack plane itself contains σ₁ and σ₂, and the crack dilates in the σ₃ direction.
7	False	Strike and dip describe the orientation of a plane. A lineation is a linear (one-dimensional) feature and is recorded by its trend and plunge instead.
8	True	Mode I fractures require a tensile effective normal stress across the crack. This can arise from a genuinely negative (tensile) regional σ₃, or from a nominally positive (compressive) σ₃ driven negative locally by elevated pore fluid pressure (hydrofracturing — see Question D) — so both signs of the nominal normal stress are possible host conditions.
9	False	For a (Newtonian) viscous material, stress is linearly related to strain rate (σ=μẁ), not to strain itself — a linear stress–strain relationship is the definition of elastic, not viscous, behaviour.
10	False	“Coaxial” means the principal strain axes do not rotate relative to a fixed external frame during progressive deformation (i.e. it describes pure shear) — it does not mean shear strain is absent. A coaxial (pure shear) deformation still has non-zero shear strain resolved on any plane not aligned with a principal axis.
11	False	Slickenside striae reliably record the slip direction (trend/plunge of the movement vector) but are one of the less reliable indicators of shear sense (which block moved which way); sense is best corroborated with stepped fibre surfaces, S–C fabric, or offset markers.
12	True	On the normal (right-way-up) limb of a fold, axial-planar cleavage is steeper than bedding. This relationship is used as a classic way-up indicator: where bedding is observed to be steeper than the cleavage that cuts it, the limb has been rotated past vertical and is overturned.
13	False	The bedding–cleavage intersection lineation (L₀=S₀∩S₁) is parallel to the fold axis (a line), not the axial surface (a plane) — a lineation cannot, by itself, describe a plane’s strike and dip.
14	False	Rock strength generally increases with increasing strain rate: a higher strain rate leaves less time for thermally-activated, rate-dependent ductile/creep processes to relax stress, so the rock behaves more brittle and requires a higher stress to fail.
15	True	For a flattening (oblate) strain ellipsoid with principal stretches S₁≥S₂≥S₃, the flattening (S₁-S₂) plane is perpendicular to the shortening direction S₃ — exactly the plane in which axial-planar cleavage develops.
16	False	The stress tensor is symmetric (σ_ij=σ_ji, from moment/angular-momentum balance), so only 6 independent components (3 normal, 3 shear) are required to fully describe it, not 9.
17	False	45° to σ₁ is indeed the plane of maximum resolved shear stress, but actual Coulomb shear fractures form at the smaller angle θ=45°−φ/2 from σ₁ (see Question D), because failure is governed by the combination of shear AND normal stress on the plane (the tangent point of the Mohr circle to the friction envelope), not by shear stress alone.
18	True	By definition, the yield point marks the transition from recoverable elastic behaviour to permanent (inelastic/plastic) strain accumulation on the stress–strain curve.
19	True	A stretching lineation is defined as the direction of maximum finite extension recorded in a rock fabric — i.e. parallel to the X (long) axis of the finite strain ellipsoid — this is the defining relationship, so it holds by definition ("always").
20	True	Shear zones are the type locality of non-coaxial (simple-shear-dominated) deformation: the progressive rotation of markers and the asymmetric fabrics used to determine shear sense both require the principal strain axes to rotate through the deformation history, which is exactly what "non-coaxial" means.

Topic. Core structural-geology vocabulary spanning stress analysis (Mohr circle, effective stress, Mode I fracture mechanics, Coulomb fracture angle), rheology (viscous vs. elastic behaviour, strain-rate/temperature dependence), fold/fault kinematics (axial-surface vs. fold-axis vs. bedding/cleavage relationships, thrust propagation direction, way-up indicators), fabric analysis (lineation conventions, intersection and stretching lineations), and shear-zone kinematics (coaxial vs. non-coaxial strain, slickenside reliability).

Key relations. Coulomb failure τ=c+σ_ntanφ, with shear-fracture angle θ=45°−φ/2 from σ₁ (not 45°); viscous flow σ=μẁ (rate-dependent, not strain-dependent); stress tensor has 6 independent components; bedding–cleavage intersection lineation ∥ fold axis; stretching lineation ∥ X-axis of the strain ellipsoid.

Why this works. Nearly every item hinges on one precise definition or a single reversed word — brittle vs. ductile (#3), σ₁ vs. σ₃ (#6), axial surface vs. fold axis (#4, #13), or the classic 45° vs. 45°−φ/2 fracture-angle trap (#17, which is exactly the geometry solved quantitatively in Question D). Items 2 & 12 are a matched pair testing the same cleavage–bedding steepness relationship in the normal vs. overturned case — getting one right and its pair wrong signals a memorized fact rather than a derived-from-first-principles understanding.

Common pitfall. Treating "coaxial" as meaning "no shear strain present" (#10) when it actually means "no rotation of the principal strain axes"; and confusing the plane of maximum shear stress (45°) with the plane on which shear fracture actually nucleates (45°−φ/2), which is smaller for any φ>0.

Source. Davis & Reynolds, Structural Geology of Rocks and Regions, 3rd ed., Ch.5–6 (stress, Mohr circle, effective stress, fracture mechanics), Ch.9 (faults, thrust propagation); Fossen, Structural Geology, 2nd ed., §3–4 (strain, coaxial vs. non-coaxial), §11–13 (rheology, folds).

Question 1 of 5: True/False Statements

Question A: True/False Statements (1 mark correct, −0.5 incorrect, blanks=0 – 20 marks total)