ON THE IDENTIFICATION OF NORMAL CROSS-CUTTING FRACTURES IN A FOLD

V. V. NIKOLAEV, Yu. N. POPOV

(Presented by Prof. A. M. Kuzmin)

The geological interpretation of materials from the study of fracturing in rocks is based on insufficiently firm theoretical principles. Much here is empirical in character and is the result chiefly of an analysis of the spatial interrelations of fractures and elements of tectonic structure. The identification of fracture systems observed at individual observation points is the principal link in the geological-structural interpretation of fracturing and the decisive factor in determining the genetic type of fractures.

In sedimentary rocks of folded regions, platforms, and transitional zones, fractures perpendicular to bedding are widely developed. The possible causes, time, and mechanism of formation of these fractures have not been fully clarified. We are probably dealing here with fractures of different genetic groups. The prevailing view of the origin of normal cross-cutting fractures, according to A. A. Belitskii [1], is the hypothesis of their tectonic origin. The regularities in the orientation of these fractures in folds make it possible to suppose that normal cross-cutting fracturing is genetically connected with folding [2, 5, 6].

Comparison of diagrams of fracture orientation is usually carried out in the coordinates of the “reduced” bed, where the \(X\) axis is directed down the dip, the \(Y\) axis along the strike of the rocks, and the \(Z\) axis is perpendicular to the bedding plane [2]. Experience in studying the fracturing of rocks and coals in the Kuzbass shows that the geometric identification of fracture systems in the coordinates of the “reduced” bed often leads to the identification of different systems as the same.

Normal cross-cutting fractures form from one to four systems, which are morphologically often very similar to one another. It is customary to consider that, both on the limbs and in the hinges of folds, one system of these fractures, called “longitudinal,” is parallel to the strike line of the rocks; the second, “transverse,” is parallel to their dip line; the two other systems, “diagonal,” are arranged diagonally with respect to the strike line of the rocks, forming angles close to \(45^\circ\) with the systems of “longitudinal” and “transverse” fractures. In the mines and quarries of the Kuzbass, in particular in the Leninskii district (Fig. 2, б), we had the opportunity to carry out a series of successive detailed observations of the fracturing of coal seams on the limbs and in the hinges of folds and directly to observe, from point to point, the change in the position of normal cross-

Fig. 1. Possible orientation of normal cross-cutting fractures in a fold:
a — stereographic projection; segments \(HH\) — projections of bedding-plane poles, \(PP_2\) — projections of the poles of “longitudinal” fractures; arcs \(D'D'\) and \(DD\) — projections of the poles of “diagonal” fractures; point \(O\) — projections of the poles of “transverse” fractures when the fold axis \(O\) is horizontal. Arcs \(H_1H_2\) — projections of bedding-plane poles, \(P_1P_2\) — projections of the poles of “longitudinal” fractures, \(D_1D_2\) and \(D_1D_2\) — projections of the poles of “diagonal” fractures; point \(O_1\) — projections of the poles of “transverse” fractures when the fold axis \(O_1\) is inclined. b — diagram of the arrangement of normal cross-cutting fractures in a syncline (plan): 1 — marker horizons; 2 — “longitudinal” fractures; 3 — “transverse” fractures; 4 — “diagonal” fractures; 5 — fold axis.

Fig. 2. Orientation of normal-cross-cutting fractures in folds:
a. Orientation diagram of “longitudinal” and “transverse” fractures in coal seams of the Malaya anticline, Prokop’evsk district, mine “Koksovaya-1” (compiled from materials of I. A. Molchanov and A. A. Belitsky [4]). Stereographic projections: 1—poles of bedding, 2—poles of fractures, 3—fold axis.
b. Orientation of normal-cross-cutting fractures in the Leninsky syncline, Leninsky district: 1—outcrops of coal seams beneath alluvium, 2—disjunctives; 3—mine shafts, 4—observation points on fracturing of rocks and coals.
c. Position of normal-cross-cutting fractures in the corner of the seam Dvoinoi, I syncline, Prokop’evsk district, mine “Maneikha” No. 11 (after observations of P. K. Kulikov [3]). Bedding elements of fractures: 1—longitudinal, 2—transverse.

cutting fractures relative to the strike line of the rocks. The above-noted spatial relationships between fractures and bedding occur in folds with rectilinear horizontal axes. On the limbs of plunging folds, “longitudinal” and “transverse” fractures are arranged diagonally with respect to the strike line of the rocks; here, “diagonal” fractures form angles close to \(45^\circ\) with the “longitudinal” and “transverse” fractures (Fig. 1a). In the hinges of such folds, as one approaches the axial plane along the bed-contour line or in another direction, the “longitudinal” fractures deviate more and more from the strike line of the rocks and, in the axial plane of the fold, become transverse with respect to the strike line of the rocks. The attitude elements of the “longitudinal” fractures change smoothly, so that in plan these fractures form fans open toward the direction of plunge of the fold axes in anticlines, or toward their rise in synclines (Fig. 1b). The “transverse” fractures, in this case, deviate more and more from the dip line of the rocks and, in the axial plane of the fold, become longitudinal with respect to the strike line of the rocks. The attitude elements of the “transverse” fractures do not change appreciably; the fractures are transverse with respect to the fold axis and are inclined toward the side opposite to the direction of its plunge. The orientation of “diagonal” normal cross-cutting fractures and their position relative to the strike and dip lines of the rocks are everywhere determined by the orientation of the “longitudinal” and “transverse” fractures, with which they are geometrically rigidly connected. Thus, fractures of one and the same system in a fold, being continued one after another in dip and strike, form complex curved surfaces, as is shown schematically in Fig. 1b.

Analysis of materials from the study of fracturing in rocks and coals in the Kuzbass shows that the orientation of normal cross-cutting fractures depends not only on the orientation of the bedding plane, but also on the spatial position of the fold axis, which accounts for the complex character of changes in the attitude elements of normal cross-cutting fractures. However, at any point of the fold the fractures are perpendicular to the bedding and at the same time the “transverse” fractures are perpendicular to, and the “longitudinal” fractures parallel to, the fold axis (Fig. 2).

The disorderliness in the orientation of normal cross-cutting fractures makes it difficult to compare fracture diagrams. Reliable identification of fracture systems in a fold can be carried out on the basis of detailed study and continuous tracing of fracture systems. The rigid geometrical connection of normal cross-cutting fractures with the axes of folds permits us, for comparison of diagrams of normal cross-cutting fracturing, to adopt such a coordinate system in which the \(Y\) axis coincides with the direction of the fold axis, the \(X\) axis is perpendicular to it and lies in the bedding plane, and the \(Z\) axis is perpendicular to the bedding plane. The use of this coordinate system may also prove useful for identifying systems of oblique cross-cutting fractures that are geometrically rigidly connected with the fold axis and the bedding plane.

REFERENCES

1. Belitskii A. A., Pakh E. M. Regularities of the tectonic structure of the Kuznetsk Basin. In: Fundamental Ideas of M. A. Usov in Geology. Publishing House of the Academy of Sciences of the Kazakh SSR, Alma-Ata, 1960, 461–492.

2. Kirillova I. V. Some problems of the mechanism of fold formation (in connection with the study of the internal structure of folded sequences). Proceedings of the Geophysical Institute, No. 6 (133). Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1949.

1. Kulikov P. K. Tectonic structure of the western part of the Prokopyevsk district of Kuzbass. Proceedings of the Tomsk Polytechnic Institute, vol. 90. Tomsk University Press, Tomsk, 1958, 174—185.

2. Molchanov I. A., Belitskii A. A. Cleavage of thick beds in the Prokopyevsk district and its influence on development workings. Proceedings of the Tomsk Industrial Institute, vol. 60, issue 1. “Krasnoe Znamya” Publishing House, Tomsk, 1939, 229—288.

3. Charushin G. V. Study of the tectonic fracturing of sedimentary rocks in the south of the Siberian Platform. Proceedings of the First All-Union Tectonophysical Conference (1957). Collection “Problems of Tectonophysics.” Edited by V. V. Belousov and M. V. Gzovskii. Gosgeoltekhizdat, Moscow, 1960, 216—229.

4. Ez V. V. On the question of fracturing in the coals of the Donbass with folded structure. Collection “Folded Deformations of the Earth’s Crust, Their Types and Mechanism of Formation.” USSR Academy of Sciences Press, Moscow, 1962, 250—264.