How to identify faults and folds in the field, and what is the difference between faults and folds

Field identification of folds: folds are generally weathered and eroded after formation, and the anticline core is prone to weathering and destruction due to joint development, which may form valley lowlands, while the oblique core may form ridges. In the field, most of the rock formations are not well exposed due to denudation and damage, and cannot be directly observed.

Observations should be made perpendicular to the strike of the strata, and fold structures can be concluded when the strata are repeated and symmetrically distributed.

Field identification of faults: mainly look at scratch marks, that is, traces of dense micro-fine grooves with a certain direction on the fault plane. Touch it in the direction of the scratch, and feel that the smooth direction is the sliding direction of the other plate.

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1. The unit direction of fold formation of the basic unit fold of fold and fault is different.

The mountains and valleys in the fault are formed by displacement along the fault plane, and half of them are caused by internal forces.

There are two kinds of structures in the folds: anticline and anticline, the anticline is more susceptible to erosion, forming a valley, and the squeezing of the chute is not easy to be eroded, forming a mountain range, which is generally the action of external force.

There are many differences between the two, but the most correct is that according to the relationship between the old and new rock formations, the center (core) of the anticline is the old rock formation, and the two wings are the new rock formations; Reverse obliquely. The crustal rock layer is fractured due to a certain strength of the force, and there is an obvious relative movement along the rupture surface is called a fault. Folds and.

2. The symmetry of folds and faults is different.

The fold is the basic unit of fold in a geological formation. That is, a bend of the rock layer in the fold change. The fold has the following elements: nucleus (center), wing (sides), apex angle (angle of junction of the wings), axial plane (imaginary face bisecting apex), hub.

Faults only produce asymmetrical repetitions of stratigraphy. The stratigraphic repetition caused by folding is symmetrical, and the stratigraphic loss formed by unconformity has an erosion surface, and some have a bottom gravel. That is, the fold flanks are repeated-symmetrical.

3. The continuity and integrity of folds and faults are different.

A positive fault with a relative decline in the upper wall.

A reverse fault that rises relatively on the wall. Reverse faults with a dip angle of less than 30° on the fault plane are also called thrust faults. The relative movement directions of the two disks of the normal fault and the reverse fault are roughly parallel to the tilt direction of the fault plane, so they are also collectively referred to as inclined slip faults.

A translational fault in which two disks move relatively horizontally along the strike of the fault, also known as a strike-slip fault (referred to as a strike-slip fault).

The length of the fault varies greatly, from a few centimeters to hundreds of kilometers, and the amount of displacement between the two disks can also vary so much. Faults are widely developed tectonic formations in tectonic movements. It's not the size.

First, the scale varies, from less than one meter to hundreds or thousands of kilometers. But all of them disrupt the continuity and integrity of the rock formation.

Summary. The formation of folds is mainly formed in the environment of extrusive stress, while faults can be formed under tensile stress and extrusion stress, normal faults are formed under tensile stress, and reverse faults are formed under extrusion stress. Sometimes, small folds can also form at faults, such as when the stratum is fractured, and then squeezed, small folds will form at the upper and lower walls of the fault.

The difference between folds, fissures, faults.

The formation of folds is mainly formed in the environment of extrusive stress, and faults can be formed under tensile stress and extrusion stress, normal faults are formed under tensile stress, and reverse faults are formed under extrusion stress. Sometimes, small folds can also form at faults, such as when the stratum is fractured, it is quietly squeezed again, and small folds will be formed at the upper and lower walls of the fault.

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Identifying faults in the field requires a wealth of experience in the field

1. There is a fault triangle.

2. There are sudden changes in the landform: steep cliffs, as well as sudden changes in the terrain along the ridge, obvious differences in landforms on both sides of the line-like boundary, the existence of straight valleys, a line of sky, and stiff bends of the river.

3. The rock strata are discontinuous or dislocated along the strike, or the boundary of the rock mass is discontinuous or dislocated.

5. Linear distribution of spring water.

6. Sudden change in formation thickness.

7. Characteristic signs such as mylonite, tectonic breccia, and friction marks can be seen.

Faults are widely developed tectonic formations in tectonic movements. It's not the size.

First, the scale varies, from small ones of less than one meter to hundreds of meters and thousands of meters. But all of them disrupt the continuity and integrity of the rock formation. In fault zones, rocks are often broken and easily eroded by weathering. Along the fault line, valleys often develop, and sometimes springs or lakes appear.

What are the forces that cause the rock formations to break and dislocate? It turns out that the strong pressure and tension generated by the movement of the earth's crust exceed the strength of the rock layer itself, which has a destructive effect on the rock. The surface where the rock strata are staggered is called the fault plane.

When the rocks in the middle of the two Birot faults rise relatively, and when the rocks on both sides fall relatively, the rocks that rise relatively are called barriers; It often forms block-shaped mountains, such as Lushan and Mount Tai in China. When the rock blocks in the middle of the two faults fall relative to each other and the rock blocks on both sides rise relatively, a graben is formed, that is, a narrow and long depression zone. China's Fenhe Plain and Weihe Valley are both grabens.

Faults are particularly important for geoscientists because the sudden movement of crustal faults along the faults is the main reason for the occurrence of **. Scientists believe that the more they study the mechanisms of faults, the more accurately they will be able to predict and even control.

Fold is one of the most common geological formations in the field, and most sedimentary rock areas have fold structures of different scales. Therefore, correctly identifying the structure of the fold is an essential skill.

When working in the field, we can only see some small, complete fold structures in a geological profile or some sections. For large pleated structures, it is difficult to see it in its entirety. In this case, the existing geological maps are first consulted and analyzed.

Then, all rock strata are traversed along the tendency of the rock strata, supplemented by the recourse along the trend of the rock strata, and the occurrence, outcrop width, new and old sequence and distribution characteristics of the rock strata are observed and determined. If the rock strata of the same age are symmetrically repeated, it indicates the presence of folds. According to the distribution characteristics of the old and new rock strata, it is judged whether it is anticline (old in the middle, new on both sides) or anticline (new in the middle, old on both sides).

Finally, the fold types are divided according to the occurrence of fold elements (axial surface occurrence, two-wing occurrence, and pivot occurrence) (Fig. 13-12).

Figure 13-12 The type of fold is determined according to the symmetrical recurrence and inclination of the rock layer.

According to Xia Bangdong, 1995).

a—upright anticline and anticline; b—anticline and anticline in the homoclinal fold.

Topography is often a direct reflection of geological formations. Some mountains are made up of monoclinic rock formations (often folded wings) and are called monocline. If the dip angle of the rock layer is gentle, and the slope on one side of the slope is gentle, and the slope on the other side is steep, it is called a one-sided mountain; The famous Purple Mountain in Nanjing is a one-sided mountain.

When the dip angle of the rock layer is large, a mountain with steep slopes on both sides is formed, which is called Pig's Back Ridge.

Anticlines and anticlines are also often characterized by high in the former and low in the latter, that is, anticline mountains and anticline valleys are formed. However, in many cases, after the rock layer folds, the top of the anticline is deformed and easily fragile, coupled with the difference in rock properties, after a long period of denudation, the terrain changes, and the anticline may be eroded into a valley, which is called an anticline valley. The topography of the slope is higher than the adjacent anticline, and is called the slope mountain. This phenomenon of terrain and structure not matching is called terrain inversion.

The age of the formation of the fold is generally between the age of the most recent strata in the rock strata that make up the fold and the age of the oldest rock strata in the overlying rock strata that do not participate in the fold. It is often bounded by angular unconformity.