B3 · Publication Volume 8

Polyphase Deformation and Overprinting

overprinting, refolding, reactivation and event chronology

Cross-cutting, refolding, crenulation, mineral growth and reactivation in a minimum event sequence
Cross-cutting, refolding, crenulation, mineral growth and reactivation in a minimum event sequence

Learning objectives

After this lesson, you should be able to distinguish structural generation from absolute age; use cross-cutting, overprinting, inclusion trails and refolding to build relative constraints; recognise transposition and reactivation; construct a directed event graph rather than a forced total sequence; correlate events between outcrops without relying on labels alone; and identify evidence that could falsify a polyphase model.

Start with a field problem

An early foliation is folded and cut by quartz veins. Some veins are themselves folded, while others cut both the folds and a crenulation cleavage. Porphyroblasts contain curved inclusion trails that appear discordant with the external foliation. A brittle fault offsets everything. How many events are required, and which relations are local rather than regional?

Labels such as D1, D2 and D3 are bookkeeping devices. “D2” at one outcrop is not automatically the same event as “D2” elsewhere, and an early fabric may be unrecognised where later transposition is intense. The defensible goal is a minimum set of relative constraints linked to observations, followed by correlation tests and numerical timing where available.

Core process model

A younger structure can cut, fold, crenulate, rotate, replace or reactivate an older one. If feature B cuts feature A, then A predates B at that locality, assuming the relation is correctly observed. If a vein cuts S1 but is folded by F2, it lies between those events. If S2 is axial planar to F2, the two may be broadly synchronous, but local timing within progressive deformation can still vary.

Overprinting creates new fabric on an older one. Crenulation cleavage arises when an earlier foliation is microfolded and differentiated. With increasing strain, the earlier fabric may be transposed into parallelism and survive only in microlithons, fold hinges, porphyroblast inclusions or low-strain domains. Absence of S1 in a high-strain zone is not proof that D1 was absent.

Refolding of F1 by F2 can create interference patterns. Their geometry depends on the angle between fold axes and axial surfaces, wavelength, amplitude, strain partitioning and erosion surface. A map pattern can suggest superposition, but oriented mesoscopic structures are needed to identify generations.

Porphyroblast inclusion trails may preserve earlier matrix fabric, grow during foliation development or rotate relative to the matrix. Interpretation requires three-dimensional inclusion geometry, mineral zoning, reaction relations and comparison across grains. A spiral-looking two-dimensional trail does not uniquely prove rotation.

Reactivation reuses an older surface under new kinematics or mechanisms. New fibres over old slickensides, brittle faults cutting mylonitic fabric, fault-rock clasts within younger gouge and opposite-sense indicators can reveal reuse. The inherited surface can localise strain without sharing the original stress field.

Evidence and measurement

Build a relation table. Rows and columns are observed features; entries state “cuts,” “folds,” “is folded by,” “contains,” “replaces,” “parallel but timing unknown,” or “no observed relation.” Include photograph or sample identifiers and confidence. This makes contradictions visible before drawing a chronological cartoon.

Map domains where each fabric is preserved. Measure S1, S2, F1 and F2 separately and document the criterion used to assign generation. At intersections, record which fabric is continuous, truncated, crenulated or mineralised. In thin section, link inclusion trails, matrix foliation, reaction rims and pressure shadows to mineral chemistry and orientation.

Numerical ages constrain specific processes. A mineral age may record growth, recrystallisation, cooling, diffusion or later disturbance. Date the textural domain, not an unlocated grain separate. Report analytical uncertainty and geological interpretation separately.

Worked example

A synthetic outcrop provides these relations:

  • S1 is folded by F2.
  • Vein V1 cuts S1 and is folded by F2.
  • S2 is axial planar to F2 and crenulates S1.
  • Porphyroblast P contains straight S1 inclusions; external S2 wraps around P.
  • Fault F3 cuts S1, S2, F2, V1 and P.
  • A second vein V3 fills part of F3 and is not offset at the exposure.

The minimum partial order is D1/S1 before V1; both before D2/F2/S2; porphyroblast growth after inclusion of S1 and before or during development of the external S2 geometry; all before F3; and F3 before V3. The exact position of P relative to V1 is unresolved because no direct relation was observed.

Represent this as a directed acyclic graph rather than forcing P and V1 into one order. A mineral rim continuous with S2 could narrow the growth interval. A V1 inclusion inside P or a P grain cut by V1 could order them directly. Until then, both sequences remain valid.

At a second outcrop, a foliation called S2 has a similar orientation but no observed crenulation relation. Correlation requires lithostratigraphic position, fold geometry, mineral assemblage, overprinting and perhaps age—not matching labels or compass attitudes alone.

Misinterpretations and uncertainty

Do not equate structural generation with one regional tectonic event or numerical time interval. Deformation may migrate, overlap or recur. Different structures can form simultaneously in different materials, while similar structures can form in separate events.

Do not treat parallel fabrics as the same generation. Later transposition can make them parallel, and cleavage refraction can rotate one fabric across layers. Conversely, one continuous fabric can vary strongly in orientation around folds.

Interference-pattern names are descriptions under ideal geometry, not unique event solutions. Erosion level and incomplete exposure can mimic patterns. Inclusion trails in one section can appear disconnected or spiral because of section position.

Practical investigation

Given a set of twelve labelled sketches or photographs, create an observation matrix and translate it into a directed graph. Identify cycles; each cycle indicates a misread relation, mixed feature label or reactivation that needs more explicit event separation. Produce all allowable event orders rather than only one.

Then choose three outcrops and test whether their D1 and D2 labels correlate. Use at least four attributes: orientation, overprinting, fold style, mineral relation, stratigraphic context or age. Map confidence and list the observation most likely to break the correlation.

Mastery check

  1. Why is D2 not automatically the same age everywhere?
  2. What minimum sequence is implied when a vein cuts S1 but is folded by F2?
  3. How can transposition hide an early fabric?
  4. Why can an event graph be better than a simple numbered list?
  5. What evidence is needed to interpret porphyroblast inclusion trails?

Sources and further reading