What Is the Epicanthic Fold?

Rock formations are generally horizontal when formed. Under the action of tectonic movement, the rock layer is bent due to stress. A bend is called fold. If a series of wave-like bending deformation occurs, it is called a fold.

1. [wrinkle]: Wrinkles

2. [fold]: Folds, that is, the rock is plastically deformed under the strong squeezing effect caused by the movement of the crust.

Artistic interpretation

1. Due to the crustal movement, the wavy bend formed by the rock formation being squeezed.

Wrinkles on the skin, for example: after all, he is old and has wrinkles on his forehead.

Fold is a geological term. Fold is the deformation that various surfaces (such as layers, textures, etc.) in the rock undergo bending under force. ^[1] It is a manifestation of the original nearly flat surface in the rock becoming a curved surface. The deformation surfaces that form folds are mostly bedding planes; the cleavage, flakes or gneiss of metamorphic rocks, and the original flow surface of magmatic rocks can also become fold planes; sometimes the joint planes and fault planes in rock formations and rocks

Folds Or the unconformity surface may deform and form wrinkles after being stressed. Therefore, folds are a common geological structure on the earth's crust. ^[1] It is most pronounced in layered rocks. Some folds are formed like squeezing a tiled newspaper from both sides to the center with both hands. The newspaper will bulge, and after the bulge is too high, the top will bend and collapse. This illustrates the effect of two forces on the formation of wrinkles. One is the horizontal compression force, and the other is its own gravity. In addition, the folds are not all swelled upwards. Those with the folds bent upwards are called anticlines; those with the folds bent downwards are called synclines. Folds are rarely formed by one force, and are often caused by multiple forces. Some folds are not obvious, and some folds are significant. Their sizes are also very different. Large ones stretch for a few kilometers or even hundreds of kilometers, while small ones can only be seen by a few centimeters or even under a microscope. The tops of many large folds are exposed to weathering and exposed to the cross section of the rock, so that the folds can be clearly seen.

Rock is

Fold diagram Bending formed by a planar structure (such as bedding, cleaving, or slicing). Individual bends are also called folds. The central part of the fold is an older stratum and the newer strata on both sides are called anticlines; the central part of the fold is a new stratum and the old strata on both sides are called a syncline. In other special cases where the formation is not inverted, the anticline is an anticline and the anticline is an anticline. The anticline and the anticline are the two basic forms of folds. The size of the folds varies greatly, ranging from hand specimens or microscopic folds under a microscope to regional folds on satellite photographs.

Fold element

Fold element Wrinkle elements are the basic components of folds and are used to describe the morphology and appearance of folds. include:

Core, the rock formation at the central part of the series of folds. The anticline core is the oldest formation in the fold, and the oblique core is the latest formation in the fold.

(2) Wing (limb) refers to the relatively straight parts on both sides of the fold. When anticline and syncline are connected, one wing is shared by both.

Several types of folds at the turning end

Hinge zone of fold refers to the curved part where the fold surface (such as rock layer) transitions from one wing to the other. The shape of the turning end includes arc-shaped, sharp-edged, box-shaped, and knee-shaped. According to this, the folds are described as arc-shaped folds (a), sharp-shaped folds (b), box-shaped folds (c), fan-shaped folds (d) and deflection (e), etc.

Hinge zone of fold refers to the line connecting the largest bending points on a single fold surface (such as a rock layer). Pivots can be straight, curved or polyline. The spatial appearance of the hub can be horizontal, inclined, or upright, and it can indicate the change in appearance of the fold in its extended direction.

Axial plane. The plane formed by the hinges of adjacent fold surfaces (such as rock layers) is called the axial plane. The axial plane is a conceived symbolic plane. The line of intersection with the ground or any other surface is called the axis trace. ^[1]

The inflection point is the boundary point between the convex part and the concave part on the continuous periodic waveform curve. That is, the point at which the curvature of the fold wings is zero.

Ridge line and groove line, the line connecting the highest point of the same dorsal fold surface is called the ridge line; on the contrary, the line connecting the lowest point of the same fold surface is called the groove line.

Wrinkle classification

Geometric classification is generally based on the position of the folds or their appearance in space and the shape of the folds.

1. Posture classification or occurrence classification is classified according to the occurrence of a single fold hub and axis:

Upright horizontal folds, the axial plane is nearly upright (inclination 80 ° 90 °), and the pivot is close to horizontal (0 ° 10 °);

Upright dumping folds, the axial plane is nearly upright, and the pivot inclination angle is 10 ° 70 °;

vertical folds, the axial plane and the hub are nearly upright;

Skew horizontal folds, the axial plane is inclined (inclination angle 20 ° 80 °), the hinge is nearly horizontal;

Skewed folds, the axial plane is tilted, and the hinge is dumped;

Lying on the fold, the axial plane and the hub are nearly horizontal;

Recumbent folds, the inclination and inclination of the axial plane and the hub are basically the same, and the inclination of the axial plane is 20 ° 80 °.

Folds 2. Morphological classification

It is divided in the form of a cross section orthogonal to the fold axis. According to the change of the thickness of the rock formations that make up the folds or the change of the curvature of each layer, it is represented by the isosceles pattern of the layers. An isosceles line is the line connecting tangent points with equal tangent angles on adjacent folds on the same wing. According to this can be divided into 3 types:

In type 1, the isosceles converge into a positive fan-shaped inward arc in the dorsal shape, that is, the curvature of the inner arc is greater than that of the outer arc. According to the degree of convergence and the thickness of the layer, it can be further divided into three sub-categories: the IA-type folds are strongly oblique, and the thickness of the fold layer is thinner at the turning end than the wings, also known as the top thin folds; IH type is Ideal parallel folds, iso-slanted vertical planes, the upper and lower planes are parallel to each other, the thickness of the folds is equal everywhere, also called equal-thickness folds; the iso-slanted lines of IC-type folds slightly converge, and the thickness of the layers at the turning end is slightly larger than the wing thick.

In Type 2, the iso oblique lines are parallel to each other. The thickness of the layer at the turning end is significantly larger than that of the wings, but the apparent thickness measured in the direction of the parallel axis is equal everywhere. The layers of this type of fold have the same curvature, and the layers have similar morphology, so they are called similar folds.

In type 3, the isosceles converge outward in an inverse fan shape in the dorsal shape. The thickness of the layer at the turning end is significantly larger than the wings, which is also called the top thick fold.

3.Classify according to the geometric relationship between the fold surfaces that make up the folds

Coordination of folds, the curved shape of each fold surface is consistent or changes regularly, such as parallel folds and similar folds;

Uncoordinated folds, the bending morphology of each fold surface is significantly different from each other, and the thickness of the layer varies very irregularly.

4: According to the shape of the turning end and the characteristics of the two wings:

Arc fold: the turning end is rounded

Box-shaped folds: The turning end is straight and the two wings are steep. At the turning point of the two wings, it is knee-shaped and looks like a box. One wing of large box-shaped folds can be called flexure, that is, the rock formation has a stepped or knee-shaped fold that slopes on one side.

Fold combination form

A series of anticline and syncline formed in the same tectonic deformation form a regular geometric pattern. The combined type of folds is a comprehensive reflection of the regional tectonic stress field, the temperature and pressure conditions during deformation, and the properties of the fold rock formation. There are three typical combinations: Alpine folds, also known as full folds. The fold belt is composed of a series of linear folds. The direction of all folds is basically the same as the direction of the fold belt. The anticline and sloping continuous waves are equally developed. Different levels of folds are combined to form huge complex anticlines and folds. Jurassic folds, also known as transitional folds. It consists of a series of near-parallel and spaced folds, with different degrees of anticline and syncline development. Typical pygmy

Folds The Luoshan-style folds are tightly closed and obvious, but the syncline between the two anticlines is gently open but not obvious. The thickness of the folds is basically unchanged. It is an equal-thickness fold. This anticline is tightly closed and wide open. The folds are also called barrier folds, such as the Huaying Mountain folds in Sichuan, China. Conversely, the diagonals are tightly closed and obvious but the anticlines between the two diagonals are gently open and often box-shaped folds, which are called trough folds. Germanic folds, also known as intermittent folds. Developed in the platform cover with very weak structural deformation, it is dominated by circular domes or oblong short-axis anticlines, and the inclination of the wings is extremely slow. They can be isolated from near-horizontal rock formations, or they can appear in groups and have regular directional arrangements, such as the short anticline.

Fold formation mechanism

The formation mechanism of the folds is closely related to its stress mode, deformation environment and deformation behavior of the rock formation. Different formation mechanisms work under different conditions, and the common ones are:

1. Longitudinal folding folds Rock formations are compressed by bedding to form folds. The rock formation is generally considered to be in an initial horizontal state before the folds, so the longitudinal bending folds are the result of the horizontal compression of the crust. Differences in mechanical properties between rock layers play a leading role in the formation of folds. If the mechanical properties of the layers in the rock series are very inconsistent, the hard layer will lose its stability and bend sinusoidally, forming equal-fold folds. The relatively soft layer acts as a medium, and flattenes at the same time Passively adjust and adapt to the bending morphology caused by the hard layer. Further squeezing, the tighter the folds of the hard layer become, the more the wings can be crushed into IC-type folds. If the mechanical properties of each layer in the rock series are relatively small and the average toughness is large, the strong and weak rock layers are collectively squashed together with the folds, and IC type to type 3 folds can be formed. The axial surface of the longitudinally folded folds is perpendicular to the extrusion direction, and the fold axis is consistent with the intermediate strain axis.

2. Lateral folds The rock layer is bent by a force that is nearly perpendicular to the layer. Since the initial state of the sedimentary rock formation is horizontal, the external force of the lateral bending fold is vertical. It can be caused by the bending of the cover caused by the rise and fall of the substrate,

Folds The overburden stratum can also be caused by the diapirism (see Diapir structure) of gravity uplift of rock or other high-plasticity layer, or it can be caused by magma upwelling. It is characterized by the folds of the strata being stretched as a whole, often forming IA-type top folds, or forming ridges on the top. When the differential elevation of the basement and the surface deposition occur simultaneously, it is the same depositional fold, the anticline appears as an underwater uplift, and the syncline appears as an underwater depression, which can cause changes in the lithofacies and thickness of the sedimentary layer.

3. Shear folds, also known as slip folds, are formed by uneven shearing of rock formations along a series of dense planes that intersect with layers. It generally occurs in ductile shear zones of relatively tough rock series (such as salt-bearing layers) or deeper layered rock series. At this time, the toughness difference between the various lithological layers is extremely small and tends to be uniform, and the average toughness of the entire rock series is relatively large. In the deformation, the lithological differences and planes are only used as signs and no longer have mechanical heterogeneity. They are passively bent due to differential shear. Its axial plane is parallel to the shear plane, so the apparent thickness of the layers measured along the axial plane are equal, which are typical similar wrinkles.

Fold flow fold

Rock under high temperature and pressure can become a solid material with high toughness and low viscosity. It deforms in a viscous flow similar to viscous fluid, forming very complex folds. Small flow folds are often seen in deep metamorphic and mixed petrified rocks. Flow folds formed under relatively simple laminar conditions are actually still shear folds, and there are still rules to follow. The complex folds formed under turbulent conditions have made it difficult to reconstruct the kinematics of the folds. It has no practical significance for analyzing the stress field to which they are subjected, but it illustrates the conditions when they were generated.

Wrinkles can also be formed by the forces of surface non-structural movement. This type of fold is limited to the surface of the earth's crust and belongs to a supergene structure. For example, the creeping structure caused by gravity on the hillside can cause the rock formation to bend in a knee shape, or even turn into a horizontal curl. Ground and water landslides, differential compaction during sedimentary diagenesis, etc., can cause different forms of folds in sedimentary rocks. Such folds are generally small in size and are often confined to one or a few rock formations.

Folds identify fold structures

Folding structures are an important part of geological structures. Folding structures of different scales exist in almost all mountainous areas composed of sedimentary rocks and partially metamorphic rocks. Small fold structures can see the complete form of one side on a geological section; large structures are often more than several kilometers to tens of thousands of meters long. The methods of studying the fold structure in the field are:

(I) Geological methods

Rock formation observation and measurement

The order of lithology, lithology, thickness, and outcrop occurrence of an area must be measured or basically cleared ^[2] in order to correctly analyze and judge the existence of folds. Then judge the anticline or syncline according to the characteristics of the symmetry and recurrence of the old and new rock formations; then determine the fold shape (including the cross section, longitudinal section, and horizontal plane) according to the occurrence of the axial plane, the occurrence of the two wings, and the occurrence of the hinge.

2. Field route inspection

The first is to use the crossing method, that is, to observe the vertical rock strata, in order to pass through all the rock strata and understand the sequence, occurrence, outcrop width, and distribution characteristics of the old and new rock strata. Second, based on the crossing method, a recourse method is adopted, that is, observation along the extension direction of a certain marker layer, in order to understand whether the two wings extend in parallel or gradually merge. These two methods can be used crosswise, or the traversal method is the main method, and the recourse method is the auxiliary method, in order to know the morphological contour of the folded structure in three dimensions.

(II) Geomorphological methods

Various soft, hard, thin, thick, and different structures often have obvious reflections on the landform. For example, hard rock formations often form high mountains, cliffs or ridges, soft formations often form gentle slopes or valleys, and so on. The landforms related to the fold structure are:

Horizontal rock formation

Some horizontal rock formations are not primitive, but are part of large fold structures, such as turning ends, fan-shaped folds or troughs, bottoms of tectonic basins, flexing turning parts, etc. Such rock formations often appear as cliffs around The gentle platform of the cliff, Fangshan and the gentle basin floor of the tectonic basin.

Monoclinic rock formation

A wing of a large folded structure or the marginal part of a structural basin often appears as a series of monoclinic rocks. Such a rock layer has a planar erosion along the rock layer in the direction of inclination, so the terrain surface is generally consistent with the slope of the rock layer; while the erosion in the anti-incline direction often exfoliates in blocks along the vertical fissures, forming steep slopes and cliffs. . Therefore, if the monoclinic rock layer has a small inclination angle (such as 20 ° ~ 30 °), a mountain with a steep slope and a gentle slope is called a single-sided mountain; if the monoclinic rock layer has a large inclination angle (such as 50 ° ~ 60 °), it will form The mountains on both sides are steep, called Pig's Back or Pig's Back.

3. Dome structure, short anticline and tectonic basin

The former two often form one or more groups of concentric circles or ellipses. If the formation of the rock formation is gentle, the inner slope is steep and the outer slope is gentle. Radial or annular water systems sometimes develop in such areas. In the tectonic basin area, there are often high mountains composed of old rock formations, and the rock formations at the bottom of the basin are gentle, and more new ones appear. For example, in the Sichuan Basin, the northern Daba Mountain is mainly composed of Paleozoic and Pre-Paleozoic rock layers, and in the center of the basin, it is mainly composed of Mesozoic and Cenozoic rock layers.

4. Horizontal folds and dump folds

In horizontal fold areas, ridges and valleys are often formed parallel to each other along two wings. Ridges and valleys in arcs or zigzags often form in fold-fold areas.

5. Anticline and syncline

The topographic advantage is basically consistent with the geological structure, that is, anticline mountains and syncline valleys are formed. However, in more cases, valleys are eroded at the anticline sites, and mountains develop at the syncline sites, that is, anticline valleys and syncline hills are formed. The phenomenon that the terrain does not match the structure is called terrain inversion.