Question 3 of 4: Structural Processes in Detail

Question C: Structural Processes in Detail (any and only 5 of 8 – 4 marks each + 1 style/clarity – 25 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 8 topics are answered below (5 of these would be selected on the actual answer sheet).

C1) Brittle structures associated with simple active folding of competent strata

As a competent layer is actively bent, its outer (convex) arc is stretched and its inner (concave) arc is shortened, generating a predictable suite of brittle structures superimposed on the fold:

(1) Outer-arc extension (tension) joints open perpendicular to bedding and parallel to the fold hinge where the outer-arc fibre stretch is greatest, typically concentrated near the crest. (2) Hinge-zone conjugate shear fractures form a symmetric X-pattern about the bedding-normal, accommodating layer-parallel shortening ahead of buckling. (3) Flexural-slip bedding-plane faults develop where adjacent layers slip past one another during flexural-slip folding, with slickenlines oriented perpendicular to the fold hinge and opposite slip senses on the two limbs. (4) Neutral-surface faulting: because bending strain reverses sign across each layer's neutral surface, small normal faults form on the outer (stretched) arc while small reverse/thrust faults form on the inner (shortened) arc of the same layer.

C2) Mohr diagram and cyclical fault pumping due to fluid pressure

Rising pore-fluid pressure P_f reduces the effective normal stress on every plane (Terzaghi: σn′ = σn − P_f), which translates the entire Mohr circle bodily to the LEFT on the σn–τ diagram without changing its radius. As P_f slowly builds (fluid influx from depth, or sealing of the fault by mineral precipitation between events), the circle migrates toward the Mohr–Coulomb envelope. The instant the circle becomes tangent to the envelope, shear (or hybrid extensional-shear) failure occurs on the optimally oriented plane: the fault ruptures, permeability spikes by orders of magnitude, the trapped fluid escapes explosively up the new/reactivated fracture, and P_f collapses — which shifts the circle back to the RIGHT, away from the envelope, restoring stability. Cementation/sealing of the newly opened fracture then allows P_f to rebuild and the whole cycle repeats: this is Sibson's fault-valve (seismic-pumping) mechanism.

The resultant vein infilling records this cyclicity directly: repeated small increments of crack, fluid escape, and mineral sealing (crack-seal texture) typically produce antitaxial or stretched fibrous quartz–carbonate veins, sometimes laminated by successive sealing events, or — where the failure mode is hybrid extensional-shear rather than pure extension — breccia/cockade veins with angular wall-rock fragments cemented by the same cyclically precipitated minerals.

C3) Four primary structures for determining younging direction

(1) Graded bedding: within a single bed deposited by a waning-energy current (e.g. a turbidite), grain size fines continuously from coarse at the base to fine at the top — younging is toward the fine end. (2) Cross-bedding: foreset laminae are truncated by the bounding surface at the TOP of each cross-set (never at the base, where they instead grade tangentially into the underlying surface) — the truncation always points toward younger beds. (3) Flute casts (and other sole marks): these scour structures form only on the original depositional BASE of a bed, so the surface bearing flute casts is always the older (bottom) contact — younging is away from the flute-marked surface, into the bed. (4) Mud cracks: desiccation cracks are open and widest at the original top (exposed) surface and taper/close downward, commonly infilled by the overlying bed's sediment — the tapering point of the wedge points toward the older material, younging is up, toward the open end.

C4) Formation of cleavage, undulose extinction, subgrain boundaries, and mechanical twinning

Cleavage forms by up to three combined mechanisms: (a) mechanical rotation of pre-existing platy minerals into alignment perpendicular to shortening; (b) pressure solution, dissolving soluble grains (quartz, calcite) at contacts oriented perpendicular to the maximum compressive stress and removing the dissolved material; and (c) syn-tectonic neocrystallization of new phyllosilicates growing with their (001) planes perpendicular to shortening. Undulose extinction is an early, low-strain indicator of intracrystalline plastic deformation: dislocation glide distributes slightly across a single crystal, so under crossed polars different sub-domains of the SAME grain extinguish at slightly different stage rotations, producing a sweeping rather than sharp extinction. Subgrain boundaries represent the next stage: with continued strain, dislocations climb and organize into low-angle (2–10°) boundaries (polygonization), partitioning the original grain into slightly misoriented subgrains — once misorientation grows large enough these become new high-angle grain boundaries via subgrain-rotation recrystallization. Mechanical (deformation) twinning (classically in calcite) is a rapid, low-temperature, diffusion-free mechanism in which part of the lattice shears instantaneously into a mirror-image orientation across a specific crystallographic twin plane — twin density and morphology are widely used as a paleostress/paleotemperature indicator.

C5) Effect of confining pressure, temperature, strain rate, and fluids on strength and ductility

Confining pressure: increasing mean stress suppresses dilatancy and crack growth, raising differential strength at failure and promoting a transition from brittle to ductile behaviour (it raises the brittle–ductile transition). Temperature: increasing temperature makes thermally activated creep (dislocation glide/climb, diffusion) easier, lowering strength and promoting ductile flow at a given stress — it lowers the brittle–ductile transition. Strain rate: a higher strain rate leaves less time for thermally activated recovery to keep pace, so higher stress is needed to achieve the same strain (power-law creep, stress ∝ strain-rate^1/n) — rapid loading favours brittle behaviour over ductile flow (the "Silly Putty" analogy: pulled fast it snaps, pulled slowly it flows). Fluids: mechanically, pore pressure reduces effective stress (σn′=σn−P_f), weakening the rock and promoting brittle failure at lower differential stress; chemically, fluids enhance pressure solution and hydrolytic weakening of silicate bonds, lowering the ductile strength and permitting crystal-plastic flow at temperatures well below the dry threshold.

C6) Four typical elements of a rock mass classification scheme

(1) Intact rock strength (UCS): higher UCS raises overall rock-mass strength, permitting a larger unsupported span and lighter support. (2) Discontinuity spacing / RQD: closely spaced discontinuities (low RQD) create a more blocky rock mass, reducing effective strength and requiring closer rock-bolt spacing or shotcrete. (3) Discontinuity condition (roughness, aperture, infill, weathering): rough, unweathered, interlocked joints retain high shear strength and stand with little support; smooth, weathered, clay-infilled joints have low friction and drive wedge/planar failures, requiring substantially more support. (4) Groundwater condition: high inflow or pressure reduces the effective normal stress on discontinuities (again via Terzaghi's principle), softens infill, and promotes instability — requiring drainage measures (weep holes, drainage galleries) in addition to structural support.

C7) Two typical fold-interference patterns for polyphase folding

Type 1 (dome-and-basin): F1 and F2 fold axes are oriented at a high angle (ideally orthogonal) to one another, and the two fold generations have comparable amplitude and wavelength; refolding an upright F1 antiform/synform pair with an orthogonal, similarly upright F2 produces alternating closed domes (where two antiformal crests coincide) and basins (where two synformal troughs coincide) — a classic "egg-carton" outcrop pattern, with no consistent relationship between the two fold axial traces. Type 3 (hook/crescent, coaxial refolding): F2's fold axis is sub-parallel to F1's (the two share close to the same trend/plunge, as in Question D-II(4) of this exam), but F2's axial surface is oriented obliquely to F1's; the F1 hinge trace is progressively bent/warped by F2 into a curved, hook- or crescent-shaped outcrop pattern rather than being cut into a grid the way Type 1 is, because the two fold axes never cross at a high angle.

C8) Large- and small-scale structures of extensional, compressional, and strike-slip terrain

Extensional terrain: large scale — listric normal faults with rollover anticlines, and metamorphic core complexes bounded by low-angle detachment faults; small/microscopic scale — boudinage (necking and segmentation of a competent layer under layer-parallel stretching), with fibrous mineral growth (syntaxial extension-vein fibres) recording the extension direction. Compressional terrain: large scale — imbricate thrust stacks and duplexes within a fold-thrust belt, and their associated foreland basin; small/microscopic scale — pressure-solution (stylolitic) cleavage, and asymmetric rotated porphyroclasts/mica fish recording top-to-foreland shear sense. Strike-slip terrain: large scale — positive or negative flower structures, and pull-apart (releasing-bend) basins; small/microscopic scale — S–C mylonitic fabrics with Riedel shear arrays (R, R′, P fractures) and sigmoidal en-echelon tension-gash veins recording the shear sense.