Geometrical features of folds and their associated minor structures Flashcards
What is reflected in the geometry of a fold?
The way it was formed
What is a cylindrical fold?
A fold that can be generated by translating a straight line with no rotation
What is the property of a cylindrical fold that is used to generate the fold?
Fold axis
What is the profile plane of a fold?
The plane that is perpendicular to the fold axis
What is the hinge line of a fold?
The line connecting points of maximum curvature
What is the inflection line of a fold?
The lines joining points of zero curvature
What is the region around the hinge line called?
Hinge zone
What is the region around the hinge zone called?
Limbs
What is the relationship between the hinge line and fold axis?
They are parallel but the hinge line is at a specfic location of the fold, the fold axis is not
What defines the boundaries of a single fold?
The portion of the folded surface that lies between two adjacent inflection points
What is the enveloping surface?
The surface that contains the crest lines or trough lines
What is the sheet dip?
The dip of the enveloping surface, it is effectively the average dip of the folded surface
What is the median surface?
The surface containing all the inflection lines
What is the axial surface?
The surface containing all the hinge lines of a nested stack (at least 2) of folded surfaces (becomes the axial plane is the surface is planar)
What is the difference between the hinge plane and axial plane?
They are parallel but the hinge plane is a specific location wheras the axial plane is not
What is the axial trace?
The line of intersection of the axial surface with any other surface of interest (usually the ground surface)
What is the difference between plunge and dip?
Plunge is measured along the axis of a fold and dip is measured along the limb
What is a synform?
Folds that close down
What is an antiform?
Folds that close up
What is a syncline?
Folds with the youngest rocks in the core
What is an anticline?
Rocks with the oldest rocks in their core
In which direction is it said that folds face?
The direction that they young within their axial plane
What is a homocline?
Uniformly tilted layering of a regional scale
What is a monocline?
A fold pair with two long horizontal limbs connected by a short inclined limb
What is a box fold?
Double-hinged fold
What is a kink fold?
Half a box fold
When is a fold symmetric?
The median surface and axial surface are perpendicular and they divide an antiform-synform pair into mirror-symmetric quarter waves
When is a fold asymmetric?
When it violate one or both (usually both) of the conditions for symmetry
Describe an asymmetric fold
The median surface and axial plane are not perpendicular and the fold limbs are of unequal length
What are the two types of asymmetric fold?
Z-fold and S-fold
What is vergence?
The direction of the antiform if you are on the long limb
When do asymmetric folds only violate one condition of symmetry?
When the limbs have different thicknesses
What happens when the limbs of an asymmetric fold are different thicknesses?
The axial plane doesn’t bisect the angle between the limbs
How is the scale of folds measured?
Amplitude and wavelength in reference to the enveloping and median surfaces
When are two amplitudes required?
When the median surface is not midway between the enveloping surfaces
What are two other measurements that are important in the formation of folds?
Arc length and layer thickness
What is a vertical fold?
Where the fold axis (and therefore the axial plane) is vertical
What is a reclined fold?
Where the plunge/plunge direction of the fold axis is the same as the dip/dip direction of the axial plane
What is used to describe the attitude of folds?
The plunge of the fold axis and the dip of the axial plane
What three quantitative measurements can be used to define fold shape?
Fold aspect ratio, fold bluntness, fold tightness
How do you work out fold aspect ratio?
Amplitude / half wavelength
What is fold bluntness?
A measure of how rounded or angular the hinge is (e.g. chevron, rounded, concentric)
What is fold tightness?
Defined by the interlimb angle (minimum angle between limbs as measured in the profile plane)
What is a parallel fold?
One that has constant orthogonal layer thickness around the fold
What is a similar fold?
One that has constnat layer thickness parallel to the axial trace
What is the dip isogon?
The line connecting points of equal dip on either side of a folded layer, classes of fold range from strongly convergent (1A) to divergent (3)
How can fold classes be shown graphically?
Orthogonal thickness / orthogonal thickness in the hinge vs. angle
What are primary and secondary structures in folds?
Primary structures provide an indication of orientation at the time of formation whilst secondary structures are produced by deformation
Are minor folds primary or secondary structures?
Secondary
How can minor folds help locate the fold hinge?
Their vergence changes across the fold hinge
Are planar fabrics/lineations primary or secondary structures?
Secondary
What are three examples of how lineations are formed?
Passive rotation of elongate crystals, mineral shape change by plasticity, oriented nucleation and growth of new grains
What is the cleavage of a rock?
The fabric formed in fine-grained rocks that can be seen with the naked eye, applies at low metamorphic grade
What is schistosity?
Coarser-grained foliations formed at higher metamorphic grades than cleavage
What is the relationship btween cleavage/schistosity and the axial surface?
Cleavage/schistosity formes parallel/sub-paralle ot the axial surface of a fold, this is described as axial planar cleavage
What happens when the cleavage is not parallel with the axial plane?
It fans about the fold in a convergent or divergent way
What is cleavage refraction?
The changing of the dip of the cleavage when it passes between lithologies, caused by the competence (resistance to flow) of the layers
What does cleavage in more competent lithologies look like?
Typically at a higher angle to the bedding
What are crenulation lineations?
The hinges of the small folds formed by the deformation of cleavage, their axial planes are crenulation cleavage
What are S0, S1, and S2?
S1 is the original bedding surface, S1 is the first cleavage, and S2 is the second cleavage (crenulation)
What are four examples of lineations formed during deformation and often lie within the cleavage plane?
Preferred orientation, small elongate concentrations, elongate mineral overgrowths, well-alighned elongate shapes
What are stretching lineations?
Mineral elongations that form parallel with the maximum extension direction of the structure on which they occur
What is pitch-and-swell?
Gentle oscillations of the thickness of the bed
What is boudinage?
When the pitch-and-swell is periodically separated
What does the shape of the boudins depend on?
The contrast in the competence with the adjacent layers
What are the three types of boudin shapes?
Block, barrel, fish-mouth
What is between separate boudins?
Secondary mineral deposits
When are boudins asymmetric?
When formed in the presence of a layer parallel shear, their rotation may be with the sense of shear
What are mullions?
Convex surfaces with intervening cusps that form at the interface of layers of very different competence
What do the cusps point to?
The more competent unit
