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 20 statements are answered below as a complete study set; on the actual answer sheet only the first 15 written would be marked. Eight statements are True and twelve are False.
| # | Answer | Justification |
|---|---|---|
| 1 | True | Coaxial refolding means the later fold shares the same principal-strain orientation as the earlier one; a single, long-lived progressive deformation with a fixed strain path (constant kinematic axes) can tighten an early fold and superimpose a coaxial second-generation geometry without any change in tectonic regime. |
| 2 | True | RQD is the percentage of core recovered in intact pieces ≥10 cm; a higher fracture frequency chops the core into shorter pieces, so fewer pieces clear the 10 cm threshold and RQD falls. |
| 3 | False | A Mohr circle represents the stress state at a point for every possible plane orientation through that point — each point ON the circle corresponds to one plane, not the whole circle to one plane. |
| 4 | True | Andersonian fault theory requires the fault plane to contain the intermediate stress axis σ2; both normal faults (σ1 vertical) and reverse faults (σ3 vertical) therefore strike parallel to the horizontal σ2 direction. |
| 5 | True | Transpressional (oblique convergent) shear zones generate dilational jogs and releasing bends that focus fluid flow, making them classic sites for orogenic/hydrothermal ore emplacement (e.g. shear-zone-hosted gold). |
| 6 | False | A face cut parallel to the joint strike exposes the joint's full true dip on the cut — if that dip daylights out of the face this is the kinematically least stable orientation (planar sliding); cutting perpendicular to strike only exposes a shallow apparent dip and is generally more stable. |
| 7 | True | By definition an ideal (perfectly) plastic material deforms at constant stress once the yield strength is reached — no strain hardening or softening — so strength beyond yield does not depend on strain magnitude. |
| 8 | False | The relative-age rule runs the other way: a joint that is unbroken and continuous where it crosses another joint is the younger one (it cuts straight through); a joint that is offset or terminated at the intersection is the older one. |
| 9 | False | Flexural-slip folding dilates the fold hinge zone (saddle reefs at anticlinal crests/synclinal troughs, e.g. the Bendigo goldfield), not the limbs — gold prospecting in actively folded terrain targets hinges, not limbs. |
| 10 | False | Solution (pressure-solution) cleavage forms by dissolving mineral material at grain contacts oriented perpendicular to maximum compressive stress and removing it in solution — this is a volume-loss (compaction) mechanism, not an increase. |
| 11 | True | As temperature and confining pressure rise toward the brittle–ductile transition, thermally activated mechanisms (pressure solution, crystal plasticity) increasingly accommodate strain in place of frictional sliding, so the effective frictional resistance of the rock mass falls. |
| 12 | True | Releasing bends and step-overs along strike-slip fault systems open pull-apart basins bounded by normal faults — graben geometry is a standard product of strike-slip systems. |
| 13 | False | In rigorous strain nomenclature, stretch S = lf/l0 is a length ratio and stays strictly positive (0<S<1 for shortening, S>1 for lengthening) — it is the related quantity extension e = S−1 that goes negative under shortening. "Stretch" itself is never negative. |
| 14 | False | Fold-thrust belts (e.g. the Canadian Rockies) are thin-skinned, shallow-crustal structures characterised by low-grade (sub-greenschist to anchizone) metamorphism; high-grade metamorphism belongs to deeper hinterland/core-zone settings. |
| 15 | False | A distributed set of conjugate shear fractures spreads strain over a rock volume in many small increments — large, localized strain is instead accommodated by a single, continuous through-going fault or shear zone. |
| 16 | False | Blocky (equant) vein infill records rapid, single-increment dilation and sealing; it is fibrous (crack-seal) vein texture that records very gradual, incremental opening tracking the extension direction. |
| 17 | False | Differential stress rises with depth only in the frictional (brittle) regime; once the brittle–ductile transition is passed, thermally activated creep weakens the rock and differential stress decreases with further depth — the classic strength-envelope curve peaks, it does not rise monotonically. |
| 18 | True | Elongation is a kinematic (geometric) quantity, not a stress state — a direction can lengthen under a fully compressive triaxial stress field (e.g. flattening strain, or mass transfer by pressure solution) with no principal stress ever going tensile. |
| 19 | False | Structures are finite-strain records of past deformation increments; the present-day in-situ stress field (measurable independently by overcoring/hydrofracture methods) commonly differs substantially from the paleostress state that produced the fabric. |
| 20 | False | Continental crust (≈35–40 km average, up to >70 km) is much thicker than oceanic crust (≈7 km average) — the statement has it backwards. |
