What is the occurrence of folds and two wings, and the two basic forms of fold structure

Think of it this way: you hold a book, hold the left and right sides of the book in your left and right hands, and hold the level. Then squeeze both hands hard at the same time, and the book will arch up, so that folds are formed.

The highest part of the arch forms a line, which is flanked by wings. It's actually very simple, you just have to look at ** to understand.

According to the occurrence of the fold axial surface, combined with the characteristics of the occurrence on both sides, it is divided into:

Upright folds: the axial plane is nearly erect, the wings tend to be opposite, and the inclination angle is nearly equal;

oblique folds: the axial surface is oblique, the wings are inclined to be opposite, and the inclination angles are unequal;

Inverted folds: the axial plane is tilted, the wings are inclined to the same, and the inclination angle can be equal or different;

Horizontal folds: the axial plane is nearly horizontal, the strata of one wing are normal, and the strata of the other wing are inverted;

Roll fold: A flat fold with a curved axial surface.

An inverted fold is an axial slope with both wings leaning towards the same fold. For the two wings of the rock formation, one wing has a normal occurrence, and the top of the formation faces upward, which is called a normal wing; The other wing is inverted with the top of the formation facing downward, which is called an inverted wing.

The contour contour line refers to the line of the tangent point where the inclination angle of the upper and lower fold surfaces of the fold layer is equal, that is, the line of the same slope point on the adjacent fold plane in the fold orthogonal profile. The type of isotilt line reflects the difference in the curvature of the inner and outer arcs of the fold layer, which is an important basis for the geometric classification of folds.

Anticline and anticline.

1.The meaning of folds: The structure of the rock Chankai layer that makes up the earth's crust is called fold structure because it is bent in a wave-like state without losing its continuity due to the strong action of tectonic stress. Fold structure is the manifestation of plastic deformation of rock strata, and it is the basic structure of extensive development of the earth's crustal surface.

2.Characteristics: It is most pronounced in layered rock formations; It is one of the most common geological formations on the earth's crust; The scale varies greatly, hand specimens - several hundred kilometers.

3.Significance: Fold is the most important tectonic phenomenon, so it is an important part of structural geology research. Relationship with minerals:

The great dip is the basin, which forms sedimentary deposits, and the tiger's eye is formed in hydrothermal filling deposits; Oil: anticline traps, the vast majority of oil discoveries in the past are related to this; Engineering geology, hydrogeology. tourism geology; Structural Geology.

4.Basic type.

A fold is a bend in the construction of a fold. It is the constituent unit of the folded structure. There are two basic types of folds: anticline and anticline.

1) Anticline. An anticline is a fold in which a rock formation rises upward. The central part is an older rock formation, which becomes new to both sides.

2) Slanted. Dip is the fold of the rock formation that is concave downward. The central part is a newer rock formation, which is getting older in turn to the sides.

If the rock is not denuded by rough fronts, the anticline becomes a mountain, and the slope becomes a valley, and only the latest strata of the era can be seen on the surface; If the fold is weathered and eroded, the anticline mountain is flattened, and the whole terrain becomes relatively flat, and even the anticline is strongly eroded to form a valley, and the anticline becomes a ridge instead. Therefore, the undulation of the terrain cannot be used as the main marker to identify the type of fold.

After the anticline and anticline were subjected to the erosion of the wind stool, strata of different ages can be seen on the surface of the ground. The understanding of anticline and anticline in the plane is determined according to the relationship between the old and the new rock strata and their distribution law. If the middle is a new stratum and the old strata appear symmetrically on both sides, it is called a dip.

The occurrence of folds is mainly determined by the axial surface and the occurrence of the pivotal celery button, therefore, the axial surface and pivot of the fold are the basic elements for the study of the occurrence and morphology of folds.

The occurrence of the axial plane, like the occurrence of the rock plane, is expressed in terms of strike, inclination and dip.

A pivot is a linear structure whose occurrence, like other linear structures, is represented by dipping (dipping angle and dipping direction) and lateral lodging (lateral lodging angle and lateral lodging). The dip angle of a linear structure is the acute angle between the structure and its horizontal projection line measured on an upright plane; The dip is the direction to which the horizontal projection line of the linear structure points to the direction of the lower end of the structural line; The lateral angle refers to the acute angle between the construction line and the strike line of the construction plane measured on the construction plane (or other geometric reference plane) where the linear structure is located; The lateral direction is the azimuth at the end of the strike line that makes up the lateral angle described above (Figure 4-4).

The axial plane is represented by a large circle of a plane (as shown in ABCDE in Figure 4-4b) or as the pole of the plane (point m in Figure 4-4b). The pivot is the projection point located on the great circle of the axial plane, which is the pole of the circle (the point in Figure 4-4b).

There is a certain geometric relationship between the dip angle ( ) and the lateral angle ( ) of the fold hub and the dip angle (a) of the structural plane where the linear structure is located, and the relationship is as follows: sin = sin ·sina.

Figure 4-4 Axial surface and pivot of the fold and its chi-flat projection.

ABCDE—axial surface; BG—hub; a—axial surface inclination; ab—the strike line of the axial plane; – pivot dip angle; bf — the projection line of the hub on the horizontal plane;

the dip direction of the hub; – the lateral angle of the hub on the axial plane;

the lateral direction of the hub on the closed surface of the folded shaft; —The point (pole) where the hub is projected on the Chiping projection map

The occurrence of the axial surface and hub of the folds, except for some small folds that can be directly measured in the field, generally large folds can be obtained by the red flat projection method according to the occurrence of different parts of the fold surface measured by the system; According to the occurrence of the fold layer on the geological map, the occurrence of the pivot and axial plane of the fold can be obtained by the method of chiping projection.

The fold hub (or fold axis) and axial surface occurrence are the basic elements for the study of fold occurrence and morphology.

The occurrence of the fold hub and all linear structures can be represented by dipping fingers (dip angle and dip direction) and lateral dipping (side angle and dip direction) (Fig. 5 6).

In addition to some small regret-type folds that can be directly measured on the outcrop, generally larger folds can be easily found by the polar chiping projection method according to the occurrence of the measured layer in the field, and can also be made according to the occurrence of the fold layer on the liquid quality map, and the occurrence of the pivot of the fold and the axial trace on the profile can be obtained according to the occurrence of the fold cross-section (orthogonal profile), and then the axial occurrence of the axial surface can be obtained by the polar chiping projection method.

Fig.5-6 The axial surface and hub of the fold and its chiping projection.