Chevron folds are a
structural
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...
feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds.
They develop in response to regional or local compressive
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
.
Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting
competences.
[ ]Turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites were ...
s, characterized by alternating high-competence sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
s and low-competence shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
s, provide the typical geological setting for chevron folds to occur.
Perpetuation of the fold structure is not geometrically limited. Given a proper stratigraphy
Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks.
Stratigrap ...
, chevrons can persist almost indefinitely.
Fold process
In response to compressional stress, geological beds fold in order to minimize dissipation of energy. Given an unconstrained bed
A bed is an item of furniture that is used as a place to sleep, rest, and relax.
Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
, folding
Fold, folding or foldable may refer to:
Arts, entertainment, and media
* ''Fold'' (album), the debut release by Australian rock band Epicure
* Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot
*Abov ...
does so by correspondingly minimizing bending and thus develops a sinusoidal geometry. In a stratigraphic sequence, beds are geometrically and physically constrained by their neighbours. Similarity must be maintained. To accommodate such constraints while maintaining sinusoidal geometry, less competent layers would need to be subjected to extensive flow. Kinked, yielding and highly localized hinges with straight limbs greatly reduce the geometrical need for deformation. Chevron folds are energetically preferred to conventional sinusoidal folds as they minimize ductile
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
flow to the expense of localized bending.
Four stages mark development of chevron folds: sinusoidal nucleation, concentric folding, straightening of limbs/sharpening of hinges, and tightening of the chevron fold.[ When inter-limb angles approach 60 degrees, frictional forces limit simple shear and flow deformation in less competent layers and favors pure shear of the whole stratigraphic complex.][ Therefore, the inter-limb angle, rapidly decreasing as a function of time given larger angles begins to stabilize as the angle nears 60 degrees. There is, however, no physical limitation on the acuteness of the fold.][
Saddle reef structures, hinge collapse and/or simply dilation of incompetent layer commonly accommodates the geometrical ''void'' created in the hinge during folding. While the incompetent layer deforms and flows, thus having complex cleavage patterns, competent layers tend to fracture radially at the hinge. These fractures are commonly infilled with crystalline ]veins
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
.[
]
Factors affecting folding
The behavior of chevron folds are effectively controlled by the characteristics of the stratigraphy
Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks.
Stratigrap ...
under deformation. Ideally, beds should alternate between high competence and low competence. The stability of chevron folding stringently requires regular thickness in the high-competence layers; conversely, regularity in low competence layers has been found to have very little effect on stability. The length of the bed and the thickness of competent beds further determines the structural stability. A 1:10 ratio between the thickness of competent beds and the length appears to be the threshold required for the formation of chevron folds. Smaller ratios require too much flow in the more ductile layers. Given high length to thickness and low high-competency to low-competency thickness ratios, irregularities in the thickness of the high-competence beds can be accommodated. However, local features appear as a consequence.[
Anomalously thick beds develop bulbous hinges, hinge collapse, hinges thrusts and/or compress via ductile flow. On the other hand, anomalously thin beds develop ]boudinage
Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.
Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as hornfels, is stretched and deformed amids ...
and/or extension via ductile flow.[
]
See also
*Chevron (land form)
A chevron is a wedge-shaped sediment deposit observed on coastlines and continental interiors around the world. The term chevron was originally used independently by Maxwell and Haynes and Hearty and others for large, V-shaped, sub-linear to par ...
References
{{Structural geology
Structural geology