The nature of the Conrad discontinuity with respect to the results of kola superdeep well drilling and the data of a deep geoelectrical survey

Abstract

The problem of superdeep drilling of the continen� tal crust was stated for the first time at a meeting in Paris in 1962; the necessity of direct study by drilling of the physical nature of the deep geophysical boundaries found by surfacebased methods, first of all, by seismic survey, became clear. Approximately at the same time, two projects started in the United States—Moho and Apollo. The Moho project was aimed at reaching the most ancient rocks of the basaltic layer by drilling the oceanic crust from the R/V Glomar Challenger. The other project, the Apollo program, as is known, was related to landing humans on the Moon, lunar core sampling, and transportation back to the Earth. In turn, the Soviet Union launched two programs: the Lunokhod automated missions, which was intended to collect lunar core samples, and the superdeep conti� nental drilling program that included drilling of ten superdeep wells on the territory of the country. The Kola Superdeep Well SG�3 was part of this extensive research into the composition and structure of the Earth's interior. The location of the SG�3 drilling was determined by two points. The first point was the location of the ancient Archean rocks of the Earth's crust in the northeastern Fennoscandian Shield resulting from a deep erosional cut; these rocks were exposed to a depth of about 10 km; they are represented, in partic� ular, by granitic gneisses of the Kola Series and Mur� mansk block (Fig. 1, 1 and 3, respectively). The sec� ond point, based on deep seismic sounding (DSS) data and the anomalously high position of the Conrad dis� continuity (K1), was identified in the area of Pech� enga, Kola Peninsula. The Conrad discontinuity was believed to be where the chemical (substantial) com� position of the Earth's crust changes: the upper gra� nitic gneiss layer of aluminumsilicate (siallic) rocks is replaced with a lower lying magnesian, basaltic (simaic) layer. The position of this boundary is usually attributed to a sharp growth of seismic wave velocity from 6.1 to 6.5 km/s. According to the DSS data, the K1 depth in the area of Pechenga is about 7 km (2)