Question 2 of 5: Definitions

Question B — Definitions (20 marks)

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.

B1 – Coaxial Strain versus Non-coaxial Strain

Coaxial strain (pure-shear-type) is progressive deformation in which the incremental strain axes stay parallel to the finite (total) strain axes throughout the deformation history — material lines parallel to the principal axes never rotate relative to a fixed external frame. Non-coaxial strain (simple-shear-type) is progressive deformation in which the incremental strain axes rotate continuously relative to the finite strain axes — the deformation carries an internal vorticity (spin), so material lines (including, in general, the instantaneous principal directions themselves) rotate progressively as straining proceeds. The practical test: in coaxial strain, a marker line initially parallel to a principal strain axis stays parallel to it at every increment; in non-coaxial strain it does not.

B2 – Fold Axis versus Fold Hinge Line

The fold axis is an abstract geometric line: for an idealised cylindrical fold it is the line which, when translated parallel to itself through space, sweeps out (generates) the entire folded surface. It is a property of the fold's overall geometry, not tied to any single folded layer. The hinge line is a concrete, physical line on one specific folded surface, connecting the points of maximum curvature (the hinge points) on that particular layer. A multilayer fold therefore has one fold axis but a separate hinge line on every folded surface within it; for a perfectly cylindrical, parallel fold the hinge lines of all layers are parallel to (and coincide in trend/plunge with) the fold axis.

B3 – Fault Scarp versus Fault Line Scarp

A fault scarp is a topographic step produced directly by recent fault movement — the physical fault plane itself forms (or nearly forms) the scarp face, so its height is a direct, largely un-eroded record of the most recent slip. A fault line scarp is a topographic step that develops later and indirectly, by differential erosion of rock units of contrasting resistance that were juxtaposed across an older — often long-inactive — fault; its height and even its exact position are controlled by erosion and rock strength, not by the original amount of slip, and the scarp may face either direction regardless of which block was originally down-thrown.

B4 – Reclined Fold versus Recumbent Fold

Both terms come from Fleuty's fold classification by axial-surface dip and hinge-line plunge. A recumbent fold has a nearly horizontal axial surface (dip 0–10°); the fold has been rotated so far that its two limbs lie roughly one above the other, and because its hinge line lies within that sub-horizontal axial surface, the hinge is sub-horizontal too (plunge 0–10°). A reclined fold has a moderately-to-steeply dipping (even overturned) axial surface, but its hinge line plunges steeply within that dipping axial surface, down close to the true dip direction (pitch/rake of the hinge on the axial surface ≈90°) — the fold has been rotated about an axis lying within its own axial surface, rather than about a horizontal axis.

B5 – Homogeneous Strain versus Inhomogeneous Strain

Homogeneous strain deforms every point of a body by the identical magnitude and orientation: originally straight lines stay straight, originally parallel lines stay parallel, and a sphere (or circle) deforms into a single, uniform strain ellipsoid (ellipse) throughout the body. Inhomogeneous (heterogeneous) strain varies from point to point: straight lines may become curved, parallel lines may cease to be parallel, and the strain ellipsoid's magnitude and/or orientation differ from place to place — the strain field around a single fold hinge is the classic natural example, though it can often be approximated as homogeneous within small enough sub-domains.