Anomalie siły ciężkości i anomalie budowy skorupy ziemskiej w Polsce

Jan Skorupa

Abstract


W artykule przedstawiono próbę wyprowadzenia związku korelacyjnego między grubością skorupy a anomaliami Bouguera dla obszaru Polski. Stwierdzono, że związek ten można opisać bardzo prostą formułą dla całej Polski, z wyjątkiem obszaru regionalnego wyżu grawimetrycznego, położonego w południowo-wschodniej części kraju. Omówiono możliwość wykorzystania wyprowadzonego związku korelacyjnego dla badania niejednorodności budowy skorupy całego kraju. Przedstawiono wstępną interpretację anomalnej budowy skorupy w obrębie obszarów wspomnianego wyżu grawimetrycznego. Zwrócono uwagę na nie całkowicie jasną sytuację tego obszaru w stosunku do przyjmowanych dziś wielkich jednostek tektonicznych.

ANOMALIES OF GRAVITY AND EARTH CRUST STRUCTURE IN POLAND

Recent developments in the studies on correlations between Earth crust thickness and Bouguer anomalies on the world, continent and region scale are discussed. Graphic comparison of such correlations are given. Moreover, some of the correlations are discussed ,in detail, similarly as differences in density parameters for the contact zone between the crust and the Upper Mantle determined by them. Moreover, there are discussed the causes of marked discrepancies between the estimations of thickness of the Earth Crust in Poland, based on results of deep seismic soundings, and the thickness data obtained by Z. Fajklewicz using the formulae proposed by R. M. Demienicka and G. P. Woollard (fide Z. Fajklewicz, 1964, 1973) and ,presented on ,two different versions of the Earth crust thickness.

A correlative equation for the thickness of the Earth crust In Poland was established on the basis of data from small-scale Bouguer anomaly maps and the results of deep seismic soundings. This relationship is valid fur almost the whole country except for the areas of anomaly thick crust (Lublin and presumably Świętokrzyski gravimetric highs).

The proposed formula for estimating Earth crust thickness is characterized by a mean error of the order of ±4.6 km for the case of single entrance values. The differences between values for the above mentioned areas of anomaly thick crust and "normal" values obtained using this formula markedly exceed the incertitude error (i.e. 3 times standard deviation). Attention is paid to previously noted role of the Lublin gravimetric high which cannot be explained by its shallow geological structure. An attempt was made to relate the Lublin high to the presence of large masses of rocks heavier than the neighbouring ones located below the top of the consolidated basement. Attention is also paid to the validity of the correlative relationship between the Earth crust thickness and Bouguer anomalies for areas differing in genesis and age (Carpathian Mts, Paleozoic and Precambrian Platforms, Sudety Mts), and its relatively high accuracy. It is further proposed to use the relationship for calculating "normal field" in gravimetry. The "normal field" would be defined by some "normalized" structure of the crust with density contrast at the Moho boundary equal 0.11 g/cm3, also obtained using the relationship. The normal field defined in this way would reflect the effects of denivellatios in the zone of the Moho discontinuity whilst the differences between the recorded values of Bouguer anomalies and normal field values would indirectly characterize the structure and homogeneity of the crust. Two versions of normal field determined in this way, as well as estimations of amplitude of the anomalies connected with Earth crust structure in eastern part of the Lublin gravimetric high are given. A possible model of "anormal masses" is given in order to illustrate their size. In addition, preliminary geological interpretation of the model is given. The attention is, however, drawn to the fact that there is no direct evidence for any connections between the area with anormal structure and currently accepted boundaries of main tectonic units of the country. On the other hand the size and location of the area implicate a necessity of redefinition of the tectonic units.

In concluding some attention is paid to the effectiveness of a complex use of the results of gravimetric and deep seismic surveys for searching for local anomalies lies in the structure of the crust even though their relations to the tectonics are still unclear. The hazard of overestimation of the significance of the data concerning local morphology of Moho discontinuity for delineating even large tectonic units is emphasized.


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