RESULTS OF TRANSFORMATIONS OF THE GRAVITATIONAL FIELD OF THE UPPER KONDOMA AREA OF MOUNTAIN SHORIA

I. P. Novikov, V. K. Rozmyslov

(Presented by the scientific seminar of the Department of Geophysical Prospecting Methods)

The Upper Kondoma ore district is located in the southern part of Mountain Shoria, at the junction of two major structures: the Uimen-Lebed depression and the Mrass anticlinorium, which in the course of their development experienced tectonic movements of different directions.

The principal components of the stratigraphic section of the district are deposits of the Proterozoic, Cambrian, and Devonian systems. Ordovician and Jurassic deposits play a much smaller role in the geological structure of the area under study, which was the result of prolonged interruptions in sediment accumulation in the history of the district’s geological development.

The largest intrusion within the district is the complexly differentiated Upper Kondoma granodiorite massif, occupying an area of about 250 sq. km. Along the western and eastern contacts of the massif runs the Kondoma-Lebed fault system. The latest movements along these dislocations occurred in post-Devonian time, since terrigenous Devonian deposits are displaced along the western branch of the faults with an amplitude on the order of 50–80 m. One of the faults of the eastern branch is complicated by a steep thrust structure. Along the thrust, Cambrian carbonate formations have been thrust over terrigenous Devonian deposits. The displacement amplitude is no less than 2000 m.

In analyzing the gravitational field, attention is drawn to the presence of a deep minimum of the acceleration of gravity from the granodiorite massif. Against the background of this minimum, zones of rupture dislocations are recorded either by high gravity gradients or by breaks and bends of isoanomalies. The dimensions of these anomalies and, accordingly, of the anomaly-forming structures, as well as their configuration and position in plan, in view of the superposition upon them of the field of intrusions of much greater intensity, cannot be established on the isoanomaly map. As is known, in potential physical fields, to which the gravitational field also belongs, the principle of superposition operates. This means that the gravitational field observed by us is an al-

...an algebraic sum of effects caused by various geological factors, that is, by the action of several geological objects differing from one another in density, shape, dimensions, and depth of occurrence. Some of these objects have a regional character and their effect is traced over large areas; others have a local character (ore bodies, local structures, etc.), and their effect is traced over limited areas. Often, in the presence of one regional object, there are several local ones.

In the analysis and interpretation of gravitational fields, in which the principle of superposition operates, great difficulties arise associated with the need to find so-called “pure anomalies” representing one or another geological interest. To find such anomalies, it is necessary to know all the components that enter into the total gravity field, for which it is necessary to use all available physical-geological data (density, dimensions, and depth of occurrence of the geological objects under study) by solving the direct problem.

The solution of the inverse problem is currently being carried out, both in the Soviet Union and abroad, on a large scale by means of field separation. The problem of field separation, in general form, amounts to increasing the “signal-to-noise” ratio and consists of the following particular tasks:

1) suppression of random measurement errors and anomalous effects caused by surface inhomogeneities;
2) isolation of the regional field;
3) attenuation of the regional field and isolation of residual (local) anomalies;
4) computation of derivatives of gravity.

The mathematical expression for the overwhelming majority of transformations can be written in the following form:

\[ Vz(x,y,o)\ \text{transf.}= \frac{1}{Q^n}\int_{-S}^{S}\int_{-L}^{L} Vz\left[(\rho-x),(\eta-y),o\right]\rho(\rho,\eta)\,d\rho\,d\eta, \]

where \(Vz(x,y,o)\) is the transform function;
\(Vz\left[(\rho-x),(\eta-y),o\right]\) is the initial function;
\(\rho,\eta\) are current coordinates;
\(\rho(\rho,\eta)\) is the weighting function;
\(Q^n\) is a coefficient playing the role of dimensionality.

It should be noted that many geophysicists have worked and continue to work on the problem of isolating regional and local fields from the total anomalous field, proceeding from various theoretical positions. Geophysicists are well acquainted with methods based on removing from the total field the regional effect obtained by averaging or upward continuation of the observed fields; graphical methods; methods based on the use of correlation methods, the theory of random functions, etc.

All these methods are based on formally stated conditions, and the expediency of applying some of them appears problematic. A different kind of method is that based on transforming anomalies into higher derivatives. The fields obtained as a result have the meaning of functions whose analytical properties are well known and can be used in geological interpretation. The following property is also of great importance: the influence of a linear regional background is completely eliminated, while a quadratic background is reduced to a constant.

In addition, the use of derivatives in the processing and interpretation of gravimetric data makes it possible to gain a deeper understanding of the analytical properties of anomalous fields and, to some extent, to overcome difficulties associated, for example, with the inability to identify and exclude extraneous influences (Malovichko, Tarunina, 1972).

As was indicated above, the transformation of gravimetric anomalies is carried out by many methods, and in applying each of them one has to make certain assumptions about the character of the fields being separated, on the basis of geological considerations. In analyzing and processing the gravitational field of the Upper Kondoma area, we made the following two types of assumptions.

1. The regional component of the gravitational field of the area varies according to a linear law, as do the components caused by the action of neighboring local geological objects. Under this type of assumption, we averaged the field and calculated residual anomalies.

2. The components of the gravitational field vary not according to a linear, but according to a more complex law: rational fractions, polynomials, etc. Under this type of assumption, we calculated the third derivative of the gravitational potential along the \(x\)-axis—\(V_{zxx}\), the second derivative along the \(z\)-axis—\(V_{zz}\), and computed variations of the gravity field \(\sigma V_z\) by the method of B. A. Andreev (Andreev, Klushin, 1962).

From an analysis of the transformed fields it follows that the fields of negative values of \(\sigma V_z\), \(\sigma V_{\text{loc}}\), and of the higher derivatives are the most widespread over the area of the region studied. Against the background of these fields, areas of positive and intense negative values of \(\sigma V_z\), \(V_{zz}\), \(V_{zxx}\), and residual fields are distinguished. The positive fields, as a rule, have an areal distribution. The nature of these anomalies is established almost unambiguously: all of them spatially coincide with the fields of distribution of Cambrian effusive formations of the Mrasskaya, Kabyrzinskaya, and Kondomskaya suites, Proterozoic sedimentary rocks, and diorites of the massif. All the rocks listed above, over which positive fields have been obtained, have relatively elevated and high densities in comparison with intrusive formations.

At the boundary of positive and negative fields, high gradients are observed almost everywhere. Such gradients indicate either the presence of a significant difference in the densities of the bordering rocks (the dip angles may also be gentle), or the presence of vertical contacts of structures of considerable thickness with a small excess density. No significant difference in the densities of the rocks of the area is observed; therefore the first assumption falls away of itself. The second assumption does not contradict the geological data. Over the greater part of the area, drilling and mining operations have established the development of steeply dipping structures with dip angles of \(70\)—\(90^\circ\).

The fields of negative values of \(\sigma V_{\text{loc}}\), \(V_{zz}\), \(V_{zxx}\), and \(\sigma V_z\) differ sharply from the positive ones in their form, dimensions, and mutual arrangement. For the most part these are narrowly local, linearly elongated zones. The strike direction of almost all zones is submeridional.

The length of most zones is quite considerable, from \(3\)—\(5\) to \(10\)—\(12\) km and more. The thickness of the zones is in some dependence on their length: the more extended zones have greater thickness, and vice versa. No relationship is observed between the linearly elongated zones and any lithological or age differences of the rocks.

connection: the same zones may pass not only, for example, through sedimentary rocks, but also through effusive formations, both in the fields of their development and along the contacts between them. The absence of such a connection, the considerable lengths of the zones, and the increased intensity of the fields within them almost unambiguously indicate that the origin of these zones is closely connected with tectonic activity.

The density of rocks, as is known, varies within wide limits. These variations in rock density are caused by various factors. Such factors are the mineralogical and chemical composition of the principal rock-forming minerals, structural and textural features, geostatic pressure, oscillatory movements expressed in the formation of intensely crushed rocks, and physicochemical processes. The available geological and geophysical material for the Upper Kondoma area makes it possible to state that the first two factors, within the limits of development of one and the same rock variety, have an insignificant effect on the density of the latter, both in plan and in section. Geostatic pressure is manifested mainly in the lower parts of the Devonian deposits of the Kazan graben, leading to a decrease in the porosity of these rocks. This process, in turn, is accompanied by slight recrystallization of the rocks and an increase in their densities.

Tectonic fracturing, crushing, and cataclasis of rocks over the area under consideration are manifested rather widely. As a rule, in areas where tectonic dislocations occur, the rocks have undergone hydrothermal alteration. Hydrothermal changes, for example, in intrusive rocks are almost always accompanied by removal of matter, formation of low-density minerals, and an increase in porosity. The decrease in density often reaches 10–20%.

Hydrothermal–metasomatic alteration of effusive rocks, as well as of intrusive rocks, leads to a decrease in density, often amounting to about \(0.1\text{–}0.2\ \mathrm{g/cm^3}\), and to a decrease in magnetic susceptibility.

On the basis of the available data, there is every reason to suppose that along most zones of tectonic fracturing and rock crushing (especially within the rocks of the intrusive massif), a weathering crust developed selectively. Transformations of rocks in the weathering crust are usually characterized by a decrease in the density of these rocks, caused by physicomechanical and geochemical processes. The former are characterized by intense crushing of the rocks, an increase in fracturing and porosity, and enhanced penetration into the fractures and pores either of groundwater or of gaseous emanations. From the geochemical point of view, a well-developed weathering crust of rocks is characterized by the formation and widespread development within it of dispersed colloidal systems with a large specific surface area, that is, with very high microporosity and low density. Weathering processes are especially characteristic of granitoids. Under the influence of these processes, microclinization of the rocks occurs and their porosity increases significantly, with a corresponding decrease in density to \(2.4\text{–}2.3\ \mathrm{g/cm^3}\) (Andreev, Klushin, 1962).

The superposition and combination of the geological factors described in zones of tectonic activity caused such a decrease in density in these zones that the latter, despite their relatively small dimensions (the width of the zones is meant), were reflected in the gravitational field of the area. It should be noted that on the gravity-intensity map

the linear-elongated zones under consideration are distinguished very weakly or are not noticeable at all. The field \(g_a\) within their limits is usually complicated by the influence of surrounding masses. The presence of these zones, their specific boundaries and dimensions in plan are established only as a result of our transformations of the original field \(g_a\) into fields of higher derivatives. Of course, one cannot conclude from this that, as a result of the recalculations, we introduced something new into our data. The fields of higher derivatives and residual anomalies obtained in the recalculations contain nothing fundamentally new in comparison with the original map \(g_a\). By means of the recalculation, only some features of the primary field \(g_a\) have been emphasized, while others, of no practical interest, have been suppressed and excluded.

In addition to positive anomalous fields of areal character and linear-elongated negative zones, against the background of the latter, within the study area, a number of negative anomalies have been identified that have an isometric form in plan. The intensity of these anomalies in all cases is higher than the intensity of the fields of areal character and of the linear-elongated zones. Their boundaries, as a rule, are distinct and are marked by high gradients. These fields are characterized by their confinement to, and close connection with, linear-elongated zones of negative fields. The occurrence of such anomalous areas within negative zones, for example at the places of their intersection or junction with other disturbances of a different strike, is problematic. The dimensions of these anomalies in plan, their form, and intensity rule out such an assumption. We explain the nature of these anomalies by the presence of bodies of small intrusions of acid composition; moreover, their age is apparently younger than the age of the rocks of the main granitoid massif and is probably related to the time of Mesozoic tectonic activity of the region.

The latter, undoubtedly (as evidenced by numerous background materials of the ZSGTU), was accompanied by activation of magmatic centers. Acid varieties of rocks, as is known, have the lowest density among intrusive formations. Gravitational fields from such bodies, in relation to the fields of the enclosing medium, will have a negative sign. The algebraic sum of these fields and the fields of the linear-elongated zones (both fields have the same sign) determined the presence of negative anomalies of isometric form. When comparing the geologic-structural scheme based on geophysical data with the geological base of the area, in many places a close connection is established between the anomalies under consideration and the outcrops of mapped rocks of acid composition.

The question may arise whether the established bodies of small intrusions are facies varieties of the granitoid massif. Such an assumption, apparently, is reliable in some cases. But in these cases the boundaries of the facies varieties of rocks, first, should be indistinct, blurred; second, these bodies also occur within the field of development of pure granites of the intrusive batholith.

LITERATURE

V. A. Andreev, I. G. Klushin. Geological Interpretation of Gravitational Anomalies. Gostoptekhizdat, 1962.

A. K. Malovichko, O. L. Tarunina. Higher Derivatives of the Gravitational Potential and Their Application in the Geological Interpretation of Anomalies. Nedra Publishing House, 1972.

Collection Problems of Processing and Interpretation of Geophysical Observations, Nos. 9, 10, Scientific Notes of Perm University, 1971, 1972.