1. Basic characteristics of folds in advanced metamorphic areas
(1) The complexity of folding behavior and geometric style, the precondition of rock folding is the anisotropy of rock, the average toughness between rock layers and the differential toughness between rock layers. The deformation habits of folded strata are different, and the differences of deformation structure levels all affect the behavior and mechanism of folds. With the uplift of the crust, the fold behavior changes from deep plastic flow fold (Figure 2 1 ~ 22) to shear flattening fold, bending flow fold and bending-sliding fold, and from bedding to oblique fold to open vertical fold. Therefore, in the advanced metamorphic area, fold facies exist in different structural levels. In addition, the multi-stage deformation further flattened and sheared the early folds, which led to the complexity of the geometric style of folds.
(2) The metamorphic solid rheological folds are widely developed. Because the early deformation of the lead zone is mainly based on the shear flow deformation mechanism of near-horizontal bedding, folds generally have the characteristics of metamorphic solid flow, and are a kind of fold structure with ductile shear rheological mechanism as the main mechanism. This kind of fold, whether bedding or bedding, forms syncline fold or rootless hook fold in rock stratum or building. This early fold is often accompanied by axial foliation, but it is also often accompanied by flow lineation.
(3) There are many kinds of fold deformation surfaces, and the deformation surfaces refer to all plane structures involved in fold deformation. The deformation surfaces involved in folds in advanced metamorphic areas are not only primary surfaces, but also secondary layered structures, foliation surfaces and foliation surfaces. There are not only primary unconformities, but also fault planes, ductile shear planes, structural detachment planes and contact surfaces of various intrusive rocks formed during the tectonic period. The rocks involved in folding include primary sedimentary rocks and volcanic rocks formed by structural metasomatism, as well as newly reformed pseudo-layered intrusive rocks. Therefore, in the same structural part of metamorphic rocks, various deformation surfaces with different properties in the same fold structural system are often involved. Because of the different properties of deformation surfaces involved in folds, their pre-existing occurrences are naturally not completely parallel, or even obviously inclined. When they are involved in the later fold at the same time, the two deformation surfaces will produce different styles of folds.
(4) Overlapping folds are widely developed, and the shape and position of early folds change due to overlapping interference, forming a characteristic structural pattern of overlapping folds in high-grade metamorphic rock areas. In different structural parts of existing folds, the conversion of adding folds or removing folds has taken place. In the fold increasing part, the rock layer is further squeezed, which causes the fold to be more closed, while in the fold removing part, the rock layer is stretched and stretched, which causes the fold to become more open.
2. Fold the geometric style
Rheological structures of metamorphic solids are mainly developed in metamorphic rocks with different qualities, which are generally characterized by strong bending, flattening and passive shear. Its basic geometric features can be summarized as follows:
(1) In metamorphic rocks, the folded dry rock often forms a compact oblique bend, and its scale depends on the thickness of the folded rock or rock series involved and the scale of the finite ductile shear zone. The scale of large recumbent folds is often thousands, dozens or even hundreds of kilometers. Small rheological folds of various configurations can be seen everywhere, and micro-scale folds are also common. Size structure often has the characteristics of multi-level combination.
(2) On a certain structural section, asymmetric fold systems with different maturity and the same reverse direction are often formed, and the basic symmetry types are mostly monoclinic symmetry and triclinic symmetry. According to the shape of the fold surface on its profile, it can be summarized as the sequence model shown in Figure 3-2- 10, which can reflect the development process of rheological folds from germination to maturity. With the gradual evolution of folds, the first formed folds even rotate to form * * * axis superimposed folds or are cut and pulled off into rootless folds or fold-structured lenses.
Figure 3-2- 10 Geometric Model of Rheological Fold of Metamorphic Solid
(According to Hansen 197 1)
W is the width of the fold and h is the height of the short wing of the fold.
The rheological properties of metamorphic solids are mostly non-cylindrical in three-dimensional space, and the hinges at all levels are obliquely distributed in waves, which is called pod-like folds, so the local hinge occurrences of main folds and secondary folds measured on the same outcrop are often inconsistent. The bending degree of fold hinge also reflects the rheological state and geometric maturity of folded rock. Such as from a flat tongue to a sheath.
(3) The shape of folds is obviously controlled by the deformation habits of rocks, and these folds generally have the structural effects of passive folds and flattening folds. The development characteristics of rheological folds are: longitudinal disharmony and transverse unevenness. Vertically, the upper and lower folds are often controlled by ductile shear zones, which makes the development of folds in each layer independent of each other. Because of the different habits of machinability, thickness and adhesion of rock strata, folds often have the characteristics of uncoordinated and semi-coordinated deformation. In the horizontal direction, the development degree of folds in different sections is not exactly the same, so in metamorphic rock series (rock formations), metamorphic solid folds often show the characteristics of in-phase and diversity.
(4) The basic feature of solid rheological folds is the development of axial foliation and tensile lineation in the middle and deep structural layers. Rheological fold and these two permeable structures, together with stone sausage and syntectonic secretory vein, form a new metamorphic solid rheological structure community. The development of related foliation and lineation often varies with the metamorphic degree of folded strata involved. In the folds of deep metamorphic strata, due to the increase of temperature and pressure, different degrees of melt appear, which makes the deformation mechanism change obviously, folds without axial foliation appear, and flow lineation is formed, which correspondingly produces high-grade metamorphic syntectonic minerals. In gneiss strata, coarse-grained minerals such as amphibole, slender plagioclase and potash feldspar are arranged directionally, which constitutes mineral lineation.
3. Types and characteristics of folds
There is no unified opinion on the classification of plastic rheological folds of high-grade metamorphic rocks. Generally, metamorphic rocks are described according to their types and fold shapes, but this is not the classification principle of geotectonics. On the basis of referring to the previous classification of plastic flow folds of metamorphic rocks, according to the characteristics of fold geometry, formation mechanism and associated fabric elements, combined with the research results of Daqingshan-Wulashan high-grade metamorphic rocks, the folds of high-grade metamorphic rocks are divided into three basic types: bedding solid flow folds, deep melt flow folds and compact flat folds.
(1) Bedding solid-state rheological fold: Bedding solid-state rheological fold refers to the rheological fold confined to layered or under construction in bedding ductile shear zones of different scales, and the deformation surface of the fold is the early lithologic layer. This kind of fold is mainly caused by nearly horizontal bedding shear deformation, which occurs in the environment where molten phase and plastic solid phase coexist in the deep structural plane of the crust. The fold shape is controlled by viscous solid rock. The scale of single fold is small, the manifold is irregular, the XZ plane manifold is asymmetric with the intestine, and the YZ plane is eyeball-shaped closed or semi-closed, and part of it is completely exposed. In space, folds are concentrated in the plastic rheological layer, but developed in the weakly deformed surrounding rock layer, so they are impermeable structural characteristics. Although most of them were transformed into various positions by late tectonic deformation, their axial planes are always parallel or nearly parallel to their stratotype interfaces. Because the formation of this fold is mainly controlled by the structure of interlayer interface, it is parallel to the constituent layers in appearance, so it is called bedding solid rheological fold.
(2) Deep melt flow folds: mainly developed in high-grade metamorphic areas, accompanied by bedding solid flow folds. Their development background is usually related to partial melting under advanced metamorphic conditions, and they are composed of light melt flow deformation, so they are called deep melt flow folds. It can be seen that the scale is generally not very large and the structural direction is unstable. Refractory solid rocks form rootless folds, passive flow folds and intestinal veins, while hard and thin refractory layers form superimposed folds and structural lens folds. When the molten light-colored body flows along the bed surface, irregular flow folds, intestinal folds and top-thick folds will be formed. If the melt is blocked or the hydraulic pressure increases during the flow, the bottom group folds in the layer will be formed. In the part where the melt contacts the solid, if it is squeezed laterally, it will flow on one side of the vein, forming a straight boundary and forming an irregular bend on the other side (Figure 23 ~ 24), which is a special type of structure of the molten vein.
(3) Tight flattening fold: Tight flattening fold is the product of near-horizontal compressive structural deformation, and the deformation surface of this fold can be original bedding or new bedding or metamorphic belt caused by metamorphic deformation. The deformation surfaces of some large folds are often the sliding sections of early structures. Compact flattening folds can be symmetrical or asymmetrical, and their main rheological direction is mostly perpendicular to the fold axis, and most of them are B-shaped folds. In the parts with relatively weak deformation, the folds are wide and gentle, and the axial plane is nearly vertical (photo 25), and the deformed surface is well preserved, so the original occurrence of the deformed surface can be determined by the enveloping surface. Closed syncline folds or rootless hook folds are formed at the parts with strong deformation, and the surface texture on the axial plane is well developed, replacing the early deformation surface (Figure 26).
4. Main contents of fold observation and research
The significance of studying small folds is to understand the relationship between small folds and large structures, so as to understand the regional structural framework. In the past, there were two main understandings: one thought that small folds were the epitome of large folds, and the style and nature of small folds directly reflected the characteristics of regional large folds; Another point of view is that small folds and large folds have certain geometric and genetic relations, but they are not closely related and basically belong to subordinate folds. Small folds seen in high-grade metamorphic areas, including superimposed folds, mostly belong to bedding slip-flow folds, and their genesis and relationship with regional tectonic framework are complex issues. From the viewpoint of metamorphic tectonic facies, structural trace combination and tectonic sequence, the small folds developed in high-grade metamorphic rocks should be divided into time first, and the small folds developed in metamorphic crustal rocks and various gneiss should not be studied according to the principle of simultaneity, but special data should be collected separately. On the basis of their respective research, we will discuss what kind of big structure they may be related to. The difficulty in reconstructing the fold pattern of metamorphic supracrustal rocks lies in the destruction and reconstruction of metamorphic supracrustal rocks after being folded by multi-stage regional granite emplacement, and the early folds are reconstructed when different degrees of ductile deformation zones are formed. Therefore, in the process of field work, as long as we can fully pay attention to this complex situation, we should pay special attention to the following aspects of data collection and research when collecting various actual data:
(1) Fold symbols and labels. In field work, the naming standards and legend codes of fold types should be unified. In outcrop observation, the formation order of folds should be judged according to the occurrence of folds. The folded codes are all represented by capital letter F, and then Arabic numerals are marked in the lower right corner of F in the order of formation. The first folding is F 1, and the second folding is F2 ... At the same time, the occurrence of different types of folds should be indicated on the geological hand map and geological map.
(2) The establishment of symbolic fold style, because of the complexity of fold morphology and the superposition of metamorphic rocks, the study of fold is rather chaotic. By establishing the symbolic fold pattern, the lateral change law of folds in the same period in the area is identified. The study of symbolic fold can also be completed by structural profile measurement. Select the section line with good outcrop, complete structural types and clear overlapping sections of folds, carefully observe and sample one by one, and finally achieve the goal.
(3) Selection and observation of anatomical region of fold structure. For different purposes and different folding periods, in order to understand the changes of folds in the study area in the same period, it is necessary to arrange different key anatomical planes for fold structural anatomy. In order to study the superposition relationship, it is necessary to take the overlap of early and late folds as the key area; In order to study the characteristics of late folds, only the wings of early folds overlap with the turning ends of late folds; In order to understand the involvement degree of early ductile shear zone in later fold and study the relationship between rock mass and fold, it is necessary to focus on specific areas. Through structural-lithologic mapping, it is necessary to select key areas for structural anatomy on the basis of extensive understanding of regional folds.
(4) Observation of fold shape and deformation surface. The observation of fold shape should be three-dimensional, that is, from different sections, through photography and sketch to record the types of folds, we can truly understand the characteristics of fold shape. For example, the sheath fold appears as a closed circle on the YZ plane, but it appears as an asymmetric shape on the XZ plane, and can show the direction of material flow. While studying the fold shape, it is necessary to know which group of plane structures the deformation surface is, because the plane structures with different properties and stages have different meanings for establishing tectonic events, although their shapes are similar.
(5) Systematic measurement of occurrence data of fold elements and other structural elements related to fold (such as axial foliation and linear structure). These are all important contents of fold field observation, and the acquisition of these data is the basis of indoor statistical research. In order to study the characteristics of fine fabrics, directional samples should be collected.