Characterisation of Folded Rocks in the Context of Exploration and Mining

Introduction of Folded Rocks

Mineralisation is commonly hosted in rocks that have undergone one or more stages of deformation. Consequently, the formation of mineral deposits commonly shows a close spatial and temporal relationship to folds in the host rocks.  In many cases the mineralisation also defines fold geometries, from the scale of large deposits down to the microscale.

Consequently, production and exploration geologists must feel comfortable in the interpretation of geometries in folded rocks and be familiar with the role that folds play in the mineralising process. This necessitates the ability to confidently document the folded rocks, which requires determination of the shape, orientation, style, age and position of the folds in question.

Some zones of folded rock will only be expressed by sporadic outcrops. This may disguise the overall scale of large folds as their exposure may be restricted to small outcrops. However, the characteristics of large folds can be inferred from outcrop-and hand specimen-scale relationships.

The analysis of folded rocks necessitates the use of a consistent terminology to describe fold morphology and a consistent terminology to ascribe different the different structural ages to fold-related elements. The first portion of this course looks at the classification of folds and fold properties, and uses a deformation nomenclature that puts fold elements in an age context with other ductile structures such as cleavages and lineations.

In addition to the information described above, there are three pieces of information that are critical to structural analysis of folded rocks:
  1. Orientation relationships of bedding and other surfaces (e.g. foliations) and of lineations.
  2. Fold vergence.
  3. Younging and structural facing.

Fold Morphology and Terminology

Folds can be documented in terms of style and orientation. The orientation of a fold is documented by measuring its axial plane and the plunge of the fold axis. The fold axis represents the line of intersection of the axial plane and the folded surface at the hinge of the fold. In essence, a fold axis is a special form ofintersection lineation. The fold axis is parallel to intersection lineations formed by the intersection of the cleavage and folded surface on the limbs of the fold.

The fold axis orientation is measured as a plunge and plunge direction. The plunge is measured in a vertical plane that contains the fold axis. The angle between a horizontal line and the fold axis in the vertical plane is the fold plunge. It is important to note the fold plunge is measured in a vertical plane containing the fold axis, NOT in the axial plane.

For upright folds with subvertical axial planes the axial plane cleavage will be approximately the same as the vertical plane containing the fold axis. However, depending on the orientation of the fold, the difference in orientation between the axial plane and the vertical plane containing the fold axis can be marked. Further information can be obtained such as the orientation of the form surface of folded layers.

Fold Geometry
Figure components comprising the geometry of a fold (left) and Diagrams to illustrate the relationship between a fold, its axial plane, and the plunge of the fold axis (right).

Fold Evolution

The folds that we see in outcrop represent the final state of strain that the rocks achieved before being exposed. Depending on the strain history, the folds will vary from gentle flexures right through to extremely tight structures, commonly with complex geometries. Folds initiate as open, relatively symmetric structures. As deformation progresses the folds tighten and a sense of asymmetry develops.

This asymmetry may become very pronounced to the point where the outcrop is dominated by extremely attenuated long limbs of folds separated by less strained short limb regions. It is much less common to see symmetric folds preserved in rocks that have experienced protracted and intense strain histories.

Characterisation of Folded Rocks
Diagram to illustrate the progressive development of a fold.

Crenulations develop in the same manner as the mesoscale folds and represent microscale manifestations of the same strain state. Crenulations will typically duplicate the same geometry as that of the portion of the fold they are located on. With increasing strain the crenulations become progressively more asymmetric to the point where the long limbs differentiate and are comprised of phyllosilicate-rich domains parallel to the fold axial plane. Once throughgoing differentiation zones have developed they accommodate much of the shearing strain that allows the movement necessary for ongoing fold development.

A very important process that operates during fold development is that of reactivation. Reactivation is a process involving accommodation of a sense of shear on inclined layering that is the opposite to that on the macroscale fold limb. This opposite sense of shear is accommodated by layering that is in a favourable orientation relative to the farfield stress and typically happens when the folded layering has been rotated to a moderate angle to 1. Thus, at intermediate stages of fold development it is common for the axial planar cleavage to accommodate a synthetic sense of shear and to aid fold development while adjacent layers will accommodate an opposite sense of shear and may even unfold some pre-folded layers.

As the folds tighten, the layers that were in favourable orientations for accommodating reactivation are progressively rotated toward the orientation of the axial plane cleavage, which accommodates a synthetic sense of shear. Consequently, reactivation is then over-ridden by synthetic shearing as the previously reactivated layers attain unfavourable orientations for accommodating the antithetic shear.

Reactivation is potentially very important for mineralisation because it operates with an opposite sense to the overall bulk movement sense on the fold limb. Thus, there will be areas in the evolving fold where tow opposing movement senses will compete and produce zones that are under tensional strain. If large variations in layer competency are present there is potential to cause brecciation and facilitate fluid inflow.

Fold Development in Zones of High Strain

High strain zones are characterised by significant intensification of the synchronously-formed foliation. Folds become progressively tighter and early-formed structures become markedly attenuated. The large changes in shape of the rocks being deformed commonly results in changes in volume as well due to shear-enhanced dissolution of material. Shape changes are also accommodated by marked extension, which occurs dominantly in the orientation of the extension/stretching lineation.

In these zones, structures such as fold axes and early-formed lineations undergo progressive rotation toward the orientation of the extension/stretching lineation. This produces sheath (or condom) folds, which are domal to extremely attenuated conical fold structures that are elongate in the orientation of the stretching lineation.

The rocks that undergo the highest strains evolve into mylonites. The term ‘mylonite’ refers to a deformation texture rather than a type of rock and so should not be used as a rock name within a stratigraphic sequence.

Mylonites are foliated and lineated rocks that show evidence for intense ductile deformation. They can occur in any rock type and can form zones from millimeters to several kilometres in width. Mylonites are recognizable by their small grain size and strongly developed, unusually regular and planar foliations. The high strain rates responsible for formation of mylonites are such that the deformed grains are unable to recrystallise and grow to greater sizes.

Mylonites contain porhyroclasts, which are remnants of resistant mineral grains that are of a larger size than the grains in the matrix. The planar fabric of mylonites is called a mylonitic foliation and commonly shows an intense linear fabric on its surface. The high strains commonly result in refolding and sheath fold formation of the mylonitic fabric during a protracted progressive deformation.

In addition to tubular sheath folds, the lineation may define fold geometries due to formation under high strain conditions and then deformation by the same strain field as the deformation progresses. Linear fabrics are usually best developed in polymineralic rocks where intense grain size reduction has taken place.

Characterisation of Folded Rocks in the Context of Exploration and Mining