On the Methodology of Tectonostructural Analysis of the Structure of Mine Fields

L. M. Petrovsky

(Presented by the Department of Geology and Exploration of Mineral Deposits)

In the process of tectonostructural analysis of the structure of tectonically complex parts of mine fields, both in the initial period of exploration and at the stage of completion of exploration work, when deciphering the character of tectonic faults one often has to deal with two circumstances.

First, even at the stage of completion of exploration of mine fields, the majority of medium-scale faults are recorded only along individual exploration lines, at best at several points of intersection within a group of closely spaced seams. In this case, as is known, it proves difficult to establish the average strike of a fault of presumed form.

Second, when constructing geological sections along exploration lines across the strike of the productive sequence, we usually obtain a picture only of the apparent displacement of parts of a given seam, which often distorts the true form of the disjunctive structure.

In the latter case, in order to determine the true character of the relative displacement of the broken parts of a seam, it is recommended [1] to construct sections across the strike of the displacer, in the plane of the most frequently encountered displacement strictly along its dip, or, more precisely, in the plane of true displacement.

However, first, the probability of distortion of the character of the relative displacement of the broken parts of a seam is, in the general case, equally inherent both in a section constructed across the strike of the seam and in a section constructed across the strike of the displacer (Fig. 1 a, b). Moreover, in the present case it is precisely in the section constructed across the strike of the seam that the picture corresponds to reality (Fig. 1 a, b) and reflects a fault in the form of a thrust (a straight thrust). In this case one may assume the simplest relative displacement of the parts of the seam along the dip of the displacer.

Second, at the stage of detailed exploration, and often also during the period of exploitation, it is practically impossible to determine either the orientation or even the direction (given a known orientation) of the actual relative displacement of the parts of the seam.

It is another matter that, under all circumstances, one should strive to determine the true elements of occurrence of tectonic faults established from exploration sections. Such a task, when only individual intersections of the seam are often available, is undoubtedly difficult to solve or, in practice, often insoluble.

At the same time, for this purpose, along with the method known from general geology [2] for determining the elements of attitude of a given plane from three points belonging to it, when an intermediate separate borehole is favorably located between the given exploration lines, the following method may be recommended. Since the distortion due to the nonperpendicularity of the section planes to the strike of the dislocator is practically absent for longitudinal and nearly longitudinal disjunctives, especially for faults with gently dipping planes of displacement, whereas for nearly transverse ones, on the contrary, it reaches very substantial values, it is possible deliberately to assume the direction of the established fault such that the line of its strike makes an angle of 50° with the strike of the seam. In this case, from the section one can determine, by the thinning or doubling of the displaced parts of the seam, the right-dipping or left-dipping character of the orientation of the disjunctive (for concordant-dipping faults).

Fig. 1. Representation of a disjunctive of the thrust-fault type:
a) in various vertical sections; b) in a section perpendicular to the strike of the seam; c) in a section perpendicular to the strike of the dislocator.

As direct observations show, in a whole series of tectonically complex industrial districts of the Kuzbass (Anzhero-Sudzhensk, Kemerovo, Prokopyevsk-Kiselevsk), of the total possible number of medium and especially small tectonic faults, under the most varied structural conditions, nearly longitudinal and diagonal disjunctives predominate almost exclusively.

For a part of substantially longitudinal and transverse disjunctives, the maximum error in determining the mean azimuth of the strike of the dislocator may be \(+30^\circ\) for nearly longitudinal ones and \(-20^\circ\) for nearly transverse ones. Further, on the basis of the properties known in the general case of dip-slip [1] disjunctives, proceeding from the value of the dip angle visible in the section along the exploration line, one can determine the mean actual dip angle of the dislocator with such an assumed strike. For this it is necessary only to make an auxiliary construction of a section across the adopted strike of the established disjunctive, taking into account corrections [2] for distortion of the true dip angle of the seam. In the absence of special correction tables, the determination both of the distorted (apparent) angle from the known true angle (of the seam or dislocator)—the direct problem—and of the true angle from the known distorted one—the inverse problem—

can be carried out quite simply graphically. For the conditions of Fig. 1 this can be illustrated by the following scheme (Fig. 2).

In this case, the error in determining the mean dip angle for nearly longitudinal faults may range from \(+\,(1—2^\circ)\) to \(+\,(4—12^\circ)\), and for nearly transverse faults—from \(-\,(4—12^\circ)\) to \(-\,(7—30^\circ)\). Thus, the form of the established disjunctives adopted at the beginning of the forecast will remain correct for all subsequently confirmed nearly longitudinal faults and for a considerable part of faults diagonal with respect to the strike of the beds.

Finally, in those cases where the relative difference between the values of the dip angles of the bed and of the displacer, for one and the same adopted form of fault, proves to be greater than the maximum value from the corresponding largest interval of errors, then the initially adopted form of the fault will be correct for all diagonal

Fig. 2. Diagram of the graphical determination of the distorted dip angle of the bed \(\alpha'\) from the known true dip angle \(\alpha\), and of the true dip angle of the disjunctive displacer \(\beta\) from the known distorted dip angle \(\beta'\).

and even for a certain part of nearly transverse faults. If, however, it proves to be equal to or less than the maximum of the largest error interval for nearly transverse faults, then the initially adopted form of the disjunctive should be regarded as provisional until its true bedding elements have been determined.

REFERENCES

1. Belitskii A. A. Classification of tectonic faults and geometric methods of studying them. Gosgeoltekhizdat, 1953.

2. Obruchev V. A. Field Geology. Vol. I. Gos. GNTI, 1932.