Question 3 of 5: Structural Interpretation Items

Question C: Structural Interpretation Items (6 marks each; answer any 4 of 5 – all 5 answered below; 24 marks nominal)

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.

C1(a). Riedel shears (R, R’) and P shears on a sinistral strike-slip fault

All three minor-shear families nucleate as a consequence of the same bulk simple-shear couple but at different orientations and different times in the shear zone’s development. R shears form earliest, oriented at a shallow angle (~15–20°) to the main fault trace, with the same (synthetic) sense of shear as the main fault — here, sinistral. R’ shears form as conjugates to R, oriented at a much higher angle (~75–80°, close to perpendicular) to the main fault, with the opposite (antithetic) sense — here, dextral. P shears form later as strain accumulates, oriented at a shallow angle on the mirror side from R (~−15–20°) but sharing R’s synthetic (sinistral) sense; with continued displacement, R and P shears link up to form the through-going principal displacement zone.

C1(b). Normal faults, reverse faults/fold axial traces, and the strain ellipse on a dextral fault

For simple shear along a fault, the instantaneous strain ellipse is oriented with its long (extensional) axis and short (shortening) axis both at 45° to the fault trace, on either side of it. For our dextral (right-lateral) fault, the extension axis trends obliquely at ~45° clockwise from the fault; normal faults / extensional (T) fractures nucleate striking parallel to this extension direction (~045° if the main fault trends 000/E–W), forming an en-echelon array. The shortening axis trends ~45° the other way (~135°), perpendicular to the extension axis; reverse faults and fold axial traces align with this shortening direction, also as an en-echelon array, perpendicular to the normal-fault set. Together these two oblique, mutually perpendicular structural families flank the main strike-slip fault — the building blocks of the positive/negative flower structures elaborated in part (c).

C1(c). Restraining and releasing bends on a dextral fault

Whether a bend in a strike-slip fault is restraining or releasing depends on the combination of the bend’s step direction and the fault’s slip sense. For a dextral fault, a bend that steps to the left forces the two fault segments to converge as slip continues, generating local transpression: uplift, folding and reverse/thrust faulting in the bend (a “pop-up” or positive flower structure). A bend that steps to the right forces the segments to diverge, generating local transtension: subsidence and normal faulting, forming a pull-apart (rhombochasm) basin bounded by the diverging fault strands (a negative flower structure). (For a sinistral fault the rule mirrors: right-stepping bends restrain, left-stepping bends release.)

C2. Buckle folds from layer-parallel shortening of a vein–sandstone–shale multilayer

Because μ_vein≈μ_sandstone>μ_shale, the veins and sandstone beds behave as the mechanically competent layers and the shale as the mechanically incompetent matrix. Under layer-parallel shortening, competent layers of similar viscosity buckle cooperatively (Biot single/multilayer buckling theory) into smooth, regular, near-sinusoidal folds with a dominant wavelength controlled by their combined thickness and the viscosity ratio μ_competent/μ_shale — approximately parallel (Class 1B) folds that maintain close to constant orthogonal thickness. The intervening shale, being far less viscous, cannot sustain its own independent buckling instability at a comparable wavelength; instead it deforms passively, flowing from the limbs into the hinge zones to accommodate the space problem created by the buckling competent layers — producing local hinge-thickening / limb-thinning (similar-style, disharmonic flow folding) in the shale that does not mirror the competent layers’ regular wavelength. The overall structure is therefore a disharmonic multilayer fold: harmonic, parallel-style buckling in the vein/sandstone, out-of-step flow folding in the shale.

C3. Foliation vs. lineation

Foliation is a penetrative planar fabric element defined by the preferred alignment of platy/tabular grains, compositional layering, or flattened grain shapes — examples include slaty cleavage (preferred orientation of phyllosilicates), schistosity, and gneissic compositional banding. Lineation is a penetrative linear fabric element, a one-dimensional direction defined within the rock — examples include a mineral stretching lineation (elongate quartz or feldspar grains, or aligned prismatic minerals such as amphibole, all parallel to the direction of maximum finite extension), and an intersection lineation formed where two planar fabrics (e.g. bedding and cleavage) intersect. A rock can carry both simultaneously: the stretching lineation typically lies within the foliation plane, so together they define the full 3-D orientation of the finite strain ellipsoid (foliation = XY flattening plane, lineation = X stretching direction within it).

C4. Joint spacing vs. bed thickness

Wider — thicker beds produce more widely spaced joints than thinner beds of the same lithology under the same extensional strain. Joint spacing is controlled by stress transfer: once a joint forms, the surrounding rock is locally stress-shadowed (unloaded) on either side of it, and a new joint can only nucleate once, moving away from the existing joint, the layer-parallel tensile stress has built back up (via shear drag across the bed-bounding interfaces) to the rock’s tensile strength. In a thicker bed, that stress-transfer distance (and hence the stress-shadow width) scales with the bed thickness, so more strain (and more lateral distance) is needed before a new joint can nucleate — giving wider average joint spacing. Thin beds re-load to failure stress over a much shorter distance, giving closely spaced joints. This joint-spacing-to-bed-thickness scaling is well documented empirically (e.g. Narr & Suppe 1991) and is the standard explanation taught alongside mechanical-layer-thickness controls on fracture spacing.

C5(A). Fold classification from the field profile sketch

C5(B). Fold class from dip isogon pattern and bed thickness

The sketch shows several nested bedding traces (the outer boundaries of the shale/sandstone/vein layers) that remain sub-parallel to one another and roughly evenly spaced as they arch through the hinge — i.e. the layers maintain close to constant orthogonal (true) thickness around the fold, rather than thickening in the hinge and thinning on the limbs. Dip isogons constructed between adjacent bedding traces (joining points of equal dip) are therefore perpendicular to bedding and converge toward the concave (core) side of the fold. This combination — isogons converging toward the core, constant orthogonal layer thickness — is the defining geometry of a Class 1B (parallel) fold, consistent with flexural-slip/flexural-flow buckling of a competent, well-bedded sequence (the same buckling style analysed quantitatively in Question C2 above).