Fold formation Flashcards
What are the three basic types of folding?
Buckling, bending, passive folding
What is buckling?
Folding that occurs when layers are loaded parallel to layering
What is bending?
Folding that occurs when layers are loaded at a high angle to the layering
What is passive folding?
Folding of marker bands that cross or are contained within a deforming unit
What does buckling require?
The folded layer to be more competent (more viscous) than the interlayers (matrix) and small irregularities (mechanical instabilities) on the layer interfaces to be nucleation sites for the folds
What is the effect of folding away from the folded layer?
The effect of folding disappears rapidly (about 1 wavelength away)
What do large viscosity contrasts lead to?
More rapid fold amplification and less layer thickening
What controls the fold wavelenth of pitch-and-swell during buckling?
The spacing between the thin sections
What happens when competent layers are close together during multilayer buckling?
They act as a thick single layer
What is harmonic folding?
When the competent layers are spaces out during multilayer buckling
What is polyharmonic folding?
Where folds with more than one wavelength form (the whole package confroms to the geometry of a controlling layer) because the layers vary in thickness and/or viscosity contrast with the matrix
What is disharmonic folding?
Layers fold independently because they are further apart than each other’s zone of influence
What is ptygmatic folding?
Where incompetent layers control the large scale deformation because they are much thicker, the fold wavelength/layer thickness relationships are regular but the axial surfaces vary greatly in orientation
Why is resistance to buckling of the whole multilayer controlled by the thickest layers?
Resistance to buckling is proportional to layer thickness cubed
When do folds need accommodation structures?
When layers start to fold early they end up with limbs that are too long or short for the ultimate fold shape, these are forced to adujst when the controlling layer starts to buckle
What are four examples of accommodation structures?
Knee-like hinge (hinge collapse), reverse faulting in the hinge, hinge thrust over, boudinaged limb
What are the two ways to model the buckling behavior of multilayers?
1; treating the individual layers as separate entities and predicting how the layers will interact when compressed, 2; treating the multilayer as a statistically homogenous but mechanically anisotropic material
What is anisotropy?
When a rock has a resistance to shearing that is different to its resistance to compression
What are two examples of materials that are classically anisotropic?
Thinly-bedded multilayers and rocks with a strongly developed penetrative fabric, such as cleavage
What does modelling aim to find?
The condition for the appearence of an instability that interrupts uniform shortning and leads to the multilayer becoming non-planar
What two features can be described by analysing buckling using anisotropy but not other methods?
Folds with very straight limbs and folds with axial planes that are oblique to the layers (even when the principal compression is along the layering)
What is a kink band?
Folds with kink-like geometries, the angularity of their hinges inhibit the use of dip isogon methods
What is used to describe kink bands?
Angles and dimensions
What does the final geometry of structures in anisotropic modelling depend on?
Degree of anisotropy and the angle at which the multilayer is compressed
How can boudinage and extensional kink bands be formed?
Layer-normal compression and active layer-parallel extension
What structure is formed very late in orogeny and what can they tell us?
Kink bands, they lack information about the orogenesis (more information as they get bigger)
What are the four stages of the development of buckles during layer parallel compression?
Pre-buckle shortning, fold initiation, fold amplification, post-buckle flattening
What stages does active buckling occur in?
Fold initiation and fold amplification, the fold continues to grow passively in stage 4
What happens during pre-buckle shortning?
Layers undergo shortning and thickening before folding is initiated. Amount of shortning depends on deformation conditions and viscosity contrast between units
What happens during fold initiation?
Irregularity in the layer initiates the onset of folding, buckling can start in different layers, or as a whole in multilayers
What happens during fold amplification and locking up?
Fold grows and layers rotate with respect to the compression direction, becoming unfavourable oriented for further buckling, the fold is now locked up
What happens during post-buckle flatning?
Continued shortning leads to passive growth, increasing the amplitude and decreasing the interlimb angle, the whole matrix and layer package thickens
How does post-buckle flattening change the geometry of a buckle fold?
Changes from class 1B to 1C
What are bend folds?
Folds that are formed as geometrically necessary folds in response to the deformation of adjacent units
What are five examples of bend folds?
Neck folds, fault-bend folds, drape folds, folds formed around intrusions/diapirs, folds formed by differential compaction
What are neck folds?
Folds betwee boudins in boudinaged layers
What are fault-bend folds?
Where hanging wall units bend as they move a fault that changes dip
What are drape folds?
Folds formed in response to faulting in the underlying basement
When does passive folding occur?
When the folded layer is a marker band that records the deformation of the unit in which it is contained, e.g. when layers enter shear zones
When is the amplification of passive folds greatest?
When the difference in mechanical properties between the layer and its host lithology is small
In what types of rocks is passive folding more prominant?
High grade rocks
What are the three types of folding mechanisms?
Flexure slip, tangential longitudinal strain (orthogonal flexure), shear (not believed to be real)
What class of folds is produced by flexural slip and tangential longitudinal strain?
Class 1B
What folding mechanisms are applicable for bending and buckling?
Flexural flow and tangential longitudinal strain
What class of folds is produced by shear folding?
Class 2
Where is the maximum slip during flexural slip?
At the inflection points, it decreases to zero towards to hinge
What are the slip planes during flexural slip?
Usually bedding or a penetrative fabric (e.g. cleavage)
What is the difference between flexural flow and flexural slip?
Flexural flow is where the slip planes are close together so the layer-parallel shear is uniformly distributed across the folded layer, flexural slip is where the slip planes are further apart and slip occurs on bedding surfaces
What are two secondary features of flexural slip folds?
Slickolites and en echelon tension gashes
What are slickolites?
Lineations on the slip plane that are oriented at a high angle to the hinge
What do tension gashes indicate?
Sense of slip
Why do lineations stay at the same anngle to the fold axis after flexural slip?
There is no distortion between the slip planes (on a stereogram, folding spreads the lineation along a small circle about the fold axis)
Describe tangential longitudinal strain (TLS)
The outer arc undergoes extension, the inner arc experiences compression, and the neutral surface between them cancels out the deformation
Where is the maximum deformation in TLS?
At the hinge, it decreases to zero towards the inflection points
What are three examples of secondary features to TLS?
Extensional fractures in the outer arc, intensified cleavage or conjugate thrusts in the inner arc
What happens to pre-existing lineations after TLS on a stereogram?
They spread within a sector
In shear folding, what are the shear planes parallel to?
The axial surface of the fold
How does shear folding effect a pre-exisiting lineation?
It is reoriented with a plane (spread along a great circle on a stereogram)
Where is the tectonic a-direction on a stereogram of shear folding?
The intersection of the great circle of lineations and the shear plane (axial plane)
What does the tectonic a-direction tell us?
If the folding is caused by shearing on a fault, this direction is the orientation of the fault slip vector
What are periclines and how are they arranged?
Elongate domes arranged in an en echelon array, formed when the fold hinge bifurcates
Give three examples of non-cylindrical folds?
Sheath folds, soft sedimentary folds, refolded folds
What are sheath folds?
Folds with hinge lines that curve more than 90deg. within the axial surface, commonly associated with a strong stretching lineation
How do sheath folds form?
Passive amplification of an initial irregularity within a layer that is subject to layer-parallel shear
Why does soft sediment deformation produce highly-non cylindrical folds?
Unlithified and water-saturated sediment is weak enough for it to respond to small changes in the direction of the forces driving the deformation
What are refolded folds?
Folds generated by two or more phases of deformation
What uncommon conditions results refolded folds being cyclindrical
When the folding events are coaxial and coplanar
What are refolded folds classified into?
Four basic fold interference patterns
Describe Type 0 fold interference pattern for refolded folds
Coacial and coplanar: the second phase of folding intensifies the first fold structure
Describe Type 1 fold interference pattern for refolded folds
The fold axes are at 90deg., both folds are upright with axial planes that are at 90deg.
Describe Type 2 fold interference pattern for refolded folds
The fold axes are at 90deg. to each other but one set of folds is recumbent and the other is upright
Describe Type 3 fold interference pattern for refolded folds
The fold axes are approximately coaxial but one set of folds is recumbent and the other is upright, forms a wavy outcrop pattern
