True/False Statements: 18-Geol-A4 Structural Geology, December 2015, Q1

Question A: True/False Statements (1 mark correct, −0.5 incorrect, blank=0 – 20 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.

All 20 statements are answered below. 7 statements are True and 13 are False.

Question A — True/False answers, all 20 statements
#	Answer	Justification
1	False	On the overturned limb of an overturned fold the beds have been rotated past vertical, so stratigraphic younging points DOWN relative to present-day up — younging is not always up; that reversal is exactly what defines “overturned.”
2	True	Standard line-length (flexural-slip) balancing — the default method for competent, foreland fold-thrust sequences — explicitly requires bed thickness (measured perpendicular to bedding) to stay constant along the section; only area-balancing techniques for ductile/detachment folds relax this requirement.
3	True	Continental crust (≈35–40 km average, locally >70 km) is much thicker than oceanic crust (≈7 km average).
4	True	A Mohr circle represents the stress state at a point for EVERY possible plane orientation through that point — a continuum of planes, i.e. infinitely many — with each point ON the circle corresponding to one plane.
5	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.
6	False	True thickness (measured perpendicular to bedding) is the shortest possible distance across a bed; any other, oblique measurement direction (vertical section, inclined borehole, etc.) gives an APPARENT thickness that is always ≥ the true thickness, never less.
7	False	Flexural-slip striae record the direction of bedding-parallel interlayer slip, which is down-dip on each limb — that slip direction is, by the mechanics of flexural slip, oriented PERPENDICULAR to the fold’s hinge line, not parallel to it.
8	True	Volcanic arcs (e.g. the Cascades, Andes, Japan) form directly above the melt-generating region of a subducting slab — they are one of the diagnostic surface expressions of active subduction.
9	False	Elongation is a kinematic (geometric) quantity, not a stress state — a material line can lengthen under a fully compressive triaxial stress field (e.g. flattening strain, or pressure-solution mass transfer) with no principal stress ever going tensile.
10	False	Flexural-slip folding dilates the fold HINGE zone (classic saddle-reef ore shoots at anticlinal crests/synclinal troughs), not the limbs — gold prospecting in actively folding terrain targets hinges, not limbs.
11	False	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, not rises.
12	False	Blocky (equant) vein infill records rapid, single-increment dilation and sealing; it is FIBROUS (crack-seal) vein texture that records very gradual, incremental, trans-tensional opening.
13	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.
14	True	A viscous (Newtonian) material has no yield threshold: strain rate is proportional to stress, so it keeps creeping under any sustained stress, however small, without needing the stress to increase.
15	False	The hinge line lies IN the axial surface, and no line within a plane can be inclined more steeply than the plane itself — so hinge-line plunge is always ≤ axial-surface dip, never greater; the statement has the inequality backwards.
16	False	“Coaxial” means the incremental and finite principal-strain axes stay parallel throughout the deformation (no progressive rotation) — it does not mean shear is absent; shear strain still exists on any plane not aligned with a principal axis (e.g. pure shear/coaxial flattening still has non-zero shear on 45° planes).
17	False	Andersonian fault theory requires the fault to STRIKE (not dip) parallel to the syntectonic σ2 — the statement as worded swaps dip for strike; since dip direction is perpendicular to strike, a fault dipping “parallel to σ2” would actually violate Anderson’s theory.
18	True	The intact-rock Mohr–Coulomb envelope is concave toward the σn-axis at very high confining pressure (curving toward a parabolic strength envelope rather than staying a straight line), so the SECANT (apparent) friction angle measured from the origin to a point on the curve decreases as confining pressure rises.
19	False	Dislocation creep is a crystal-plastic, effectively constant-volume (isochoric) deformation mechanism; it is pressure-solution (a diffusive mass-transfer mechanism), not dislocation creep, that produces volume reduction.
20	True	Mechanical stratigraphy: stiffer, more brittle layers cannot accommodate bulk strain by ductile flow and so develop closely spaced joint sets to take up the same strain, whereas softer, more ductile units accommodate more strain aseismically and develop fewer, more widely spaced joints.

Topic. Core structural-geology vocabulary spanning stress analysis (Mohr circle, friction, apparent friction-angle curvature), rock-mass engineering (RQD, mechanical stratigraphy), fold/fault kinematics (flexural slip, overturned younging, Andersonian faulting), rheology (viscous flow, dislocation creep), balanced-section theory, and true-vs-apparent geometric measurement.

Key relations. RQD = ∑(core pieces ≥10 cm)/(total run length) ×100%; Andersonian fault rule — fault STRIKE ∥ σ2; true thickness t = apparent thickness × cos(obliquity angle) ≤ apparent thickness; hinge-line plunge ≤ axial-surface dip (geometric necessity, line-in-plane); flexural-slip slip direction ⊥ fold axis.

Why this works. Each statement hinges on ONE precise definition or mechanism — confusing a plane with the locus of all planes (#4), swapping strike for dip in Anderson’s rule (#17), or reversing a relative-age rule (#13) are exactly the traps a professional licensing exam is designed to probe; several pairs (true thickness vs. apparent, hinge plunge vs. axial dip, flexural-slip direction vs. fold axis) are direct geometric consequences of a line/section being oblique to a reference plane.

Common pitfall. Assuming “apparent” measurements can go either way relative to the true value (#6, #15) when the geometry only ever allows apparent ≥ true; and confusing “coaxial” (no rotation of strain axes) with “no shear strain present” (#16) when the two concepts are independent.

Source. Davis & Reynolds, Structural Geology of Rocks and Regions, 3rd ed., ch.5 (stress, Mohr circle, friction), ch.9 (faults, Anderson’s theory), ch.11 (folds, flexural slip, balanced sections); Fossen, Structural Geology, 2nd ed., §3–4 (strain, coaxial vs. non-coaxial) & §13 (crystal-plastic mechanisms); Bieniawski, Engineering Rock Mass Classifications (RQD, mechanical stratigraphy).

Question 1 of 5: True/False Statements

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