18-Geol-A4 Structural Geology · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A4 Structural Geology, 2013-Dec. Open book; any non-communicating calculator permitted; 3 hours. The paper is printed as five lettered mega-questions (A–E): Question A instructs "answer 15 of these 20" T/F items, Question B "any and only 8 of the following" (14 term pairs), Question C "any and only 5 of the following" (9 essay topics), Question D "ONE and ONLY ONE of D-I or D-II," and Question E "ONE and ONLY ONE of E-I or E-II."
Reference texts: Davis & Reynolds, Structural Geology of Rocks and Regions, 3rd ed. (fold and fault mechanics, stress and strain, Mohr circle analysis); Fossen, Structural Geology, 2nd ed. (rheology, shear zones, fold classification, finite strain); Marshak & Mitra, Basic Methods of Structural Geology (stereonets, block diagrams); Hoek, Practical Rock Engineering; Bieniawski, Engineering Rock Mass Classifications (RQD/RMR, rock mass strength).
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.
All 14 pairs are distinguished below (8 of these would be selected on the actual answer sheet). "Allocthon/Autocthon" are spelled this way in the paper but are standard misspellings of Allochthon/Autochthon.
An autochthon is a rock mass that remains in (or very near) its original site of formation/deposition, structurally beneath or adjacent to any overlying thrust sheets. An allochthon is a rock mass that has been tectonically transported a significant distance from its original site — typically carried on a thrust or detachment — so its present neighbours are not its original stratigraphic or structural neighbours.
An anticline is a fold that closes upward with the oldest beds in its core, convex-up in profile, bounded by two limbs dipping away from the hinge. A monocline is a one-limbed, step-like flexure that connects two otherwise horizontal (or gently dipping) panels of strata at different structural levels — it has no closure and no second limb, so it is a local steepening rather than a true fold.
Brittle deformation accommodates strain by fracturing and frictional sliding, is favoured by low temperature/confining pressure and high strain rate, and causes a loss of cohesion across discrete surfaces. Ductile deformation accommodates strain by continuous, distributed flow (crystal-plastic or diffusive mechanisms), is favoured by higher temperature/confining pressure and lower strain rate, and preserves cohesion throughout the deforming volume.
Cleavage is a planar fabric produced by the preferred (parallel) alignment of platy/tabular minerals or by pressure-solution seams, generally developing perpendicular to the shortening (maximum compressive) direction. A stretch lineation is a linear fabric produced by elongated mineral grains, boudinaged/stretched aggregates, or fibrous overgrowths, oriented parallel to the direction of maximum finite extension — often found ON the cleavage plane, but recording a different (linear, not planar) strain axis.
Cohesion (c) is the shear strength a material retains at zero normal stress — an intrinsic bonding/cementation term, independent of confinement. Friction is the component of shear resistance that scales with normal stress through the coefficient tanφ, arising from grain-to-grain or surface-to-surface contact resistance; together they define the Mohr–Coulomb envelope τ = c + σn·tanφ.
The equal-angle (stereographic/Wulff) projection preserves angular relationships between lines and planes exactly, so true angles can be measured directly anywhere on the net, but it distorts area (points crowd disproportionately near the primitive). The equal-area (Schmidt/Lambert) projection preserves area — essential for unbiased statistical contouring of many data points (fabric density diagrams) — but angular relationships are only approximately preserved off-centre.
Both are brittle, cohesive-to-friable fault rocks produced by mechanical fragmentation, but they sit at different points on the comminution/depth spectrum. Fault breccia is coarse-grained (>30% visible, often angular clasts ≥2 mm), typically weakly cohesive to friable, and forms at shallow crustal levels. Cataclasite is finer-grained and more thoroughly comminuted (dominant matrix, smaller clast fraction), is cohesive, and forms via cataclastic flow at somewhat greater confining pressure/depth than breccia.
A fault-bend fold is a single fold generated where a thrust sheet is forced to ramp up and over a bend (a ramp–flat transition) in an underlying detachment, producing a geometrically predictable kink-fold above the bend. A fold-thrust belt is the entire regional structural assemblage — many stacked thrust sheets, duplexes, and fault-bend/fault-propagation folds — that together accommodate large-scale crustal shortening across an orogen; a fault-bend fold is one structural element that can recur many times within a fold-thrust belt.
The hinge line is the line of maximum curvature on a single folded surface (one specific bedding horizon). The (axial) fold plane is the surface that contains the hinge lines of ALL the folded layers through the fold stack, connecting them into one composite reference surface that describes the fold's overall attitude (upright, recumbent, plunging, etc.).
A horst is a relatively uplifted, elongate crustal block bounded on both sides by normal faults that dip AWAY from the block. A graben is the complementary structure: a relatively down-dropped block bounded by normal faults that dip TOWARD each other (into the block) — horsts and grabens typically occur together in extensional fault arrays.
In parallel folding (Class 1B), bed thickness measured PERPENDICULAR to bedding stays constant from limb to hinge — the layers are concentric and the fold geometry cannot be extended indefinitely at depth without a detachment. In similar folding (Class 2), thickness measured PARALLEL to the axial surface stays constant instead — the fold profile repeats identically at every structural level, with the layer thickening in hinges and thinning on limbs (true perpendicular thickness is NOT preserved).
RQD (Rock Quality Designation) is a single, narrowly defined index: the percentage of a core run recovered as intact pieces ≥10 cm long. RMR (Rock Mass Rating) is a composite classification that combines RQD with several other parameters (intact UCS, joint spacing, joint condition/roughness/infill/weathering, groundwater condition, and joint orientation relative to the excavation) into one overall rock-mass quality rating used directly for support design.
Pure shear is a coaxial strain: the principal strain axes stay fixed in orientation throughout the deformation (no bulk rotation), as in symmetric flattening/stretching. Simple shear is a non-coaxial, constant-volume, rotational strain in which one set of material lines (parallel to the shear plane) is never rotated while everything else progressively rotates toward the shear direction as strain accumulates — this rotation of the finite strain axes relative to the incremental strain axes is the defining difference from pure shear.
True dip is the maximum possible inclination of a plane, measured in the vertical section drawn perpendicular to the plane's strike. Apparent dip is the (always smaller) inclination of that same plane measured in any OTHER vertical section not perpendicular to strike; apparent dip approaches zero as the section approaches parallelism with strike, per tan(apparent dip) = tan(true dip)·cos(angle between section and strike).
