B3 · Publication Volume 8

Shear Zones and Kinematic Indicators

fabrics, S–C structures, porphyroclasts and shear sense

Shear-zone foliation, shear bands, asymmetric objects and independent tests
Shear-zone foliation, shear bands, asymmetric objects and independent tests

Learning objectives

After this lesson, you should be able to define a shear zone at an appropriate scale; distinguish foliation, shear bands and stretching lineation; recognise common asymmetric indicators; relate simple-shear amount to displacement gradient; separate finite strain, vorticity and shear sense; and design an oriented, multi-indicator test of shear-zone kinematics.

Start with a field problem

A 60 m wide mylonitic zone contains a strong foliation, quartz ribbons, asymmetric porphyroclasts and narrow surfaces that cut the foliation. Most indicators suggest top-to-the-east movement, but rotated veins in one domain suggest the opposite. Is the minority evidence wrong, local back-rotation, conjugate shearing, a later overprint or a sign that the regional interpretation is oversimplified?

Kinematic indicators are not votes detached from position. Their reliability depends on section orientation, object–matrix competence, strain magnitude, vorticity, boundary geometry and overprinting. The task is to map where each indicator occurs, determine which event it belongs to and ask whether one velocity field can explain the distribution.

Core process model

A shear zone is a tabular or anastomosing region across which displacement varies continuously or through closely spaced surfaces. Its boundaries are defined by a strain or fabric gradient at the scale of interest. Brittle faults, semibrittle zones and ductile shear zones form a continuum of localisation styles; the term does not require one depth or mechanism.

In ideal simple shear, displacement u parallel to the zone changes across coordinate y, and engineering shear is


\gamma=\frac{\partial u}{\partial y}.

For a uniform zone of width w and total boundary-parallel displacement \Delta u, \gamma=\Delta u/w. Natural zones are rarely uniform, so the local gradient matters. Pure-shear components can thicken or thin the zone, and rigid-body rotation can accompany deformation.

The foliation commonly labelled S records flattening or finite strain in the deforming matrix. C surfaces or shear bands may lie closer to the shear-zone boundary and cut S at an acute angle. Their geometry can indicate shear sense when the viewing direction and event relation are correct. Other indicators include asymmetric porphyroclast systems, mica fish, oblique grain-shape fabrics, displaced markers, asymmetric pressure shadows, quarter structures, vein arrays and fold vergence.

Shear sense is the sign of relative motion. Vorticity describes the rotational component of flow relative to stretching. Finite strain integrates the history. A high finite strain does not specify one vorticity, and an instantaneous kinematic field does not provide total displacement without duration and spatial integration.

Indicators can reverse locally near rigid objects, zone margins, bends or interacting strands. A robust interpretation uses several independent types distributed across oriented sections and anchored by regional offsets or piercing relations.

Evidence and measurement

Measure zone-boundary orientation, width, foliation, lineation, shear bands, fold axes, veins and object asymmetry along transects. Record distance across the zone so that gradients are visible. Use sections parallel to the stretching lineation and perpendicular to foliation for many shear-sense criteria, but verify the criterion-specific section.

At thin-section scale, distinguish recrystallised matrix from inherited clasts and alteration. Record grain-boundary shape, subgrains, crystallographic preferred orientation, mineral reactions and the relation between indicator and foliation. A single elegant porphyroclast is not a population.

Map truncations and cross-cutting relations. A shear band that cuts and offsets the dominant foliation may be late within the same progressive event or may belong to a later event. Mineral ages can date growth or cooling rather than total shearing; link dated domains to microstructure.

Worked example

A zone is 60 m wide and marker correlation suggests 90 m of boundary-parallel displacement distributed approximately uniformly. The first-order simple-shear estimate is


\gamma=\frac{90}{60}=1.5.

For \mathbf{F}=\begin{bmatrix}1&1.5\\0&1\end{bmatrix}, principal stretches are 2.0 and 0.5, giving a finite ellipse axial ratio of 4.0. Its long axis lies about 26.6° from the shear direction in the current configuration.

Field measurements show that most displacement occurs in a 15 m central band. If 70 m of displacement occurs there, local \gamma is about 4.7, far greater than the whole-zone average. The central fabric should therefore not be compared directly with an average strain estimate.

Twenty-eight oriented indicators comprise displaced markers, S–C relations and porphyroclasts. Twenty-three support the same sense, three are ambiguous and two opposite-sense indicators lie beside a competent lens. The defensible conclusion is a dominant regional shear sense with local perturbation to test, not unanimous proof and not an unweighted 23-to-2 vote.

Misinterpretations and uncertainty

Never report shear sense without viewing direction. “Top to the right” reverses when the section is viewed from the opposite side. Convert local page directions to geographic movement.

S–C angles are not a universal strain gauge. They vary with mineralogy, partitioning, transposition and progressive rotation. Porphyroclast tails can grow, recrystallise or be cut by later bands. Opposite asymmetry can arise from local flow around objects.

Zone width depends on the threshold used to define its boundary and on exposure. Displacement may predate the observed fabric or continue on a narrower fault. Dividing total offset by present width is a model, not a direct local strain measurement.

Practical investigation

Create a transect table across a synthetic shear zone with distance, foliation, lineation, grain size, shear-band orientation and indicator sense. Plot each variable against distance. Define boundaries using two different thresholds and calculate how average \gamma changes.

On oriented photographs, classify indicators by type and independence. Exclude any image whose viewing direction is unknown from the sense calculation, but retain it as descriptive evidence. Construct one progressive-deformation model and one reactivation model that explain the same minority indicators.

Mastery check

  1. How is a shear zone boundary defined?
  2. Why can whole-zone average shear differ from central-band shear?
  3. What is the difference between shear sense, vorticity and finite strain?
  4. Why must viewing direction accompany a kinematic indicator?
  5. How should a minority of opposite-sense indicators be handled?

Sources and further reading