True/False Statements: 18-Geol-A4 Structural Geology, May 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	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.
2	False	The relationship is reversed: folding is a ductile/plastic process (continuous, cohesive deformation), while fracturing is the brittle process (loss of cohesion, discrete failure surfaces).
3	False	A stress state that plots 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.
4	True	Effective stress σ′ = σ − P_p; raising pore pressure P_p reduces both effective principal stresses by the same amount, translating the whole Mohr circle to the left (toward the failure envelope) without changing its radius.
5	True	Mode I (extension/opening) 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 that is driven negative locally by elevated pore fluid pressure (hydraulic fracturing) — so both signs of the nominal normal stress are possible host conditions.
6	False	This is self-contradictory: plastic deformation is by definition permanent (non-recoverable). A perfect-plastic material deforms at constant stress beyond yield with no elastic recovery of the plastic strain.
7	False	Strike-slip systems commonly generate local compression at restraining bends/stepovers (transpression: folds, thrusts, positive flower structures) and local extension at releasing bends/stepovers (transtension: normal faults, pull-apart basins) — see Question C1(c) below.
8	False	Slickenside striae reliably record the slip direction (the trend/plunge of the movement vector) but are one of the less reliable indicators of the sense of shear (which block moved which way) — slickenfibre steps and stepped surfaces can be mis-read and are best corroborated with S-C fabric, offset markers, or other independent criteria.
9	False	The opposite is the general rule: axial-planar cleavage parallels the steep axial surface of an upright fold, while bedding on the limbs dips at a shallower angle away from the hinge (same logic as item 1) — so cleavage is typically the steeper of the two on the limbs.
10	True	Because axial-planar cleavage and bedding are both folded about (and cleavage is parallel to) the hinge line, their line of intersection (L₀ = S₀∩S₁) is parallel to the fold axis — the classic, reliable bedding–cleavage intersection lineation used to recover hinge orientation.
11	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.
12	True	For a flattening (oblate) strain ellipsoid with principal stretches S₁≥S₂≥S₃, the flattening plane is the plane containing the two longest axes (S₁-S₂), perpendicular to the shortening direction S₃ — this is exactly the definition of the foliation plane in a flattening fabric.
13	False	A Mode I (extension) crack forms with its plane containing σ₁ and opens (dilates) in the σ₃ direction — i.e. perpendicular to σ₃, not σ₁. Loosely, the crack plane is parallel to σ₁, not perpendicular to it.
14	True	By definition, the yield point marks the transition from recoverable elastic behaviour to permanent (inelastic/plastic) strain accumulation on the stress–strain curve.
15	False	Mylonites form by crystal-plastic (ductile) deformation in deep shear zones and are the archetypal ductile fault rock; brittle fault rocks are instead cataclasite, fault breccia and gouge.
16	False	Thrust faults are low-angle reverse faults, classically dipping less than about 30° (often much less); steep dips characterise high-angle reverse or normal faults, not thrusts.
17	True	Coaxial deformation histories are those in which the principal strain axes do not rotate with respect to a fixed external reference frame during progressive deformation — this non-rotational strain path is, by definition, pure shear (as opposed to the rotational, non-coaxial path of simple shear).
18	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 for this specific lineation type.
19	False	The brittle–ductile transition depends strongly on mineralogy/rock type (e.g. quartz transitions at a lower temperature than feldspar) as well as on confining pressure, temperature, strain rate and pore fluid pressure.
20	True	This is the classic basin-inversion scenario: a pre-existing normal fault (typically formed at a moderate-to-steep dip, ~50–60°) is reactivated under a later compressive regime; because the inherited fault plane is steeper than a newly-formed reverse fault would be (~25–35°), the reactivated structure is an anomalously high-angle reverse fault.

Topic. Core structural-geology vocabulary spanning stress analysis (Mohr circle, effective stress, Mode I fracture mechanics), rheology (perfect plasticity, strain-rate/temperature dependence, brittle–ductile transition), fold/fault kinematics (axial-surface vs. bedding dip, cleavage–bedding relationships, thrust geometry, fault reactivation), fabric analysis (intersection lineations, stretching lineations, flattening plane), and shear-zone kinematics (coaxial vs. non-coaxial strain, slickenside reliability).

Key relations. Effective stress σ′=σ−P_p; Coulomb failure τ=C+σ_ntanφ; flattening plane = S₁-S₂ plane of the strain ellipsoid; bedding–cleavage intersection lineation ∥ fold axis; stretching lineation ∥ X-axis of the strain ellipsoid.

Why this works. Each item hinges on one precise definition or mechanism — reversing which process is brittle vs. ductile (#2), reversing which stress axis a Mode I fracture opens relative to (#13), or reversing the age-vs-dip logic of upright-fold cleavage/bedding relationships (#1, #9) are exactly the traps a professional licensing exam is designed to probe. Several pairs (items 1&9, 5&13) are direct geometric consequences of the SAME underlying relationship (axial-planar fabrics parallel the axial surface, which is steeper than the limbs it bisects), so 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" (item 17) when it actually means "no rotation of the principal strain axes" — a pure-shear (coaxial) deformation still has non-zero shear strain resolved on any plane not aligned with a principal axis; and assuming pore pressure changes the differential stress (items 4, 5) when it only translates the Mohr circle without changing its radius.

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

Question 1 of 5: True/False Statements

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