18-Geol-A4 Structural Geology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A4 Structural Geology, 2013-May. Open book; any non-communicating calculator permitted; 3 hours.
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); 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 (10 of these would be selected on the actual answer sheet).
A porphyroblast is a large mineral crystal that grew in place during metamorphism (e.g. garnet, staurolite), syn- to post-tectonic new growth set in a finer-grained matrix. A porphyroclast is instead a large relict grain — typically an original igneous or detrital mineral (e.g. feldspar) — that survived cataclasis and dynamic recrystallization within a mylonitic shear zone, sitting in a finer, dynamically recrystallized matrix. The distinction is growth (blast) versus survival/breakdown (clast) of a coarse grain.
True dip is the maximum angle of inclination of a plane, measured in the vertical section perpendicular to strike. Apparent dip is the angle of inclination measured in any other vertical section, and is always less than or equal to the true dip — it approaches zero as the section approaches parallelism with strike.
Parallel folds (Class 1B, concentric) keep constant orthogonal (bed-normal) thickness around the fold; dip isogons converge toward the core, and because the geometry cannot be maintained indefinitely with depth, parallel folds typically die out or require a detachment at depth. Similar folds (Class 2) keep constant thickness measured parallel to the axial plane instead — the same fold shape is repeated at every structural level — and can in principle persist to any depth; they are typical of ductile, incompetent multilayers rather than competent, brittle ones.
Elastic stiffness (e.g. Young's modulus E) is the ratio of stress to recoverable elastic strain — resistance to elastic deformation before yield. Ductility is the capacity for large permanent (plastic) strain after yield without loss of cohesion. The two are independent: a material can be elastically stiff yet brittle (fails suddenly near yield), or elastically soft yet highly ductile.
Cohesion (c) is the shear-strength intercept at zero normal stress in the Mohr–Coulomb criterion — the intrinsic bonding strength of intact material, independent of normal stress. Friction is the normal-stress-dependent component of shear resistance, τ = σn·tanφ, arising from resistance to sliding on an existing surface; it grows linearly with normal stress and, unlike cohesion, is generally close to zero cohesion-wise for a pre-existing joint.
A fault-bend fold is a single fold produced where a thrust sheet is forced to ramp up (or down) over a non-planar fault surface, its geometry kinematically dictated by the fault-surface shape (Suppe's fault-bend-fold theory). A fold-thrust belt is the entire regional orogenic province — an imbricate system of many thrust faults and their associated folds (fault-bend, fault-propagation, and detachment folds stacked in duplexes and imbricate fans) across a mountain front. The fault-bend fold is one structural element found within a fold-thrust belt.
Pure shear is coaxial, non-rotational strain: the principal strain axes stay fixed in orientation through progressive deformation (e.g. symmetric flattening/stretching). Simple shear is non-coaxial, rotational strain: the principal strain axes progressively rotate relative to a fixed material line, while one set of planes (the shear plane) stays parallel to itself — the deformation geometry characteristic of shear zones.
Strike-slip is the true net-slip vector measured along strike on the fault surface — a kinematic quantity describing actual relative displacement. Strike separation is the apparent offset of a marker measured on a map, i.e. the horizontal distance between the two traces of an offset feature along strike; it equals the true strike-slip only in the special case of a vertical marker with no dip-slip component, so separation and slip should never be equated without checking the marker geometry.
A listric fault is a single fault surface, concave-upward in cross-section, that flattens with depth (steep near surface, soling into a sub-horizontal detachment); it produces a rollover anticline in its hanging wall. Imbricate faulting is instead a system of many closely spaced, sub-parallel faults splaying from a common sole, each carrying a slice of the same stratigraphy stacked like roof shingles (an imbricate fan or duplex) — a multi-fault architecture, not a single curved-surface geometry.
The fold (axial) plane is the surface that bisects a fold, connecting the hinge lines of every folded layer stacked through the multilayer, and dividing the fold into (ideally) symmetric halves. The hinge line is the line of maximum curvature on one single folded surface (one bedding layer). The axial plane is therefore the locus that contains/connects the many parallel hinge lines belonging to the different layers of the same fold.
A horst is an elevated, relatively up-thrown fault block bounded by normal faults that dip away from each other (diverging downward), standing structurally higher than its neighbours. A graben is a down-dropped block bounded by normal faults dipping toward each other, forming a linear depression (e.g. a rift valley) — horsts and grabens are complementary, alternating features of the same extensional fault array.
RQD (Rock Quality Designation) is a single-parameter index — the percentage of drill core recovered in intact pieces ≥10 cm — that reflects fracture spacing alone. RMR (Rock Mass Rating, Bieniawski) is a composite classification combining RQD with intact-rock UCS, discontinuity spacing, discontinuity condition (roughness/infill/weathering), groundwater condition, and a discontinuity-orientation adjustment, to give an overall 0–100 rock-mass-quality rating used for excavation and support design. RQD is one of six inputs that feed RMR, not a substitute for it.
The equal-angle (Wulff) net preserves true angular relationships everywhere on the net (correct for angle-measurement and two-/three-point construction problems) but distorts area, so plotted point density is not statistically meaningful. The equal-area (Schmidt/Lambert) net preserves area (equal solid angles on the sphere plot as equal areas on the net) at the cost of a small angular distortion, making it the correct choice for plotting and contouring large populations of orientation data (e.g. joint or fold-axis statistics).
Cleavage is a planar fabric produced by preferred alignment of platy minerals (or spaced pressure-solution seams), representing the plane of flattening (perpendicular to shortening) in the finite strain ellipsoid. A stretch lineation is a linear fabric defined by elongate mineral grains, aggregates, or stretched clasts aligned parallel to the direction of maximum extension (the X-axis of the strain ellipsoid); it commonly lies within the cleavage plane and records the transport/extension direction, e.g. in a shear zone.
