Transcontinental crustal sections of the U.S.S.R. and adjacent areas

Abstract

Transcontinental sections of the earth's crust and Upper Mantle from the Kuril Islands to the Carpathians, and along other principal directions, in all about 15 000 km long, show the following features:(1) The depth of an asthenosphere horizon agrees well with the position of a high-conductivity layer in the Upper Mantle.(2) Hypocenters of earthquakes are located in fault zones. In the zone of the Kuril Island arc, they plunge into the Upper Mantle to depths of hundreds of kilometers, dipping towards the continent.(3) Boundary velocities along the Mohorovičić discontinuity are not dependent on its depth, for they are always in the range 8.0–8.2 km/s; velocities within the crust are correlated with the depths of layers; in the roots of high mountains a layer of 7.5–7.8 km/s has been distinguished near the base of the crust.(3) The crustal thickness in folded areas is in a certain accordance with the altitude of the surface of the earth's solid cover, with Bouguer anomalies and amplitudes of neotectonic movements. It varies from 8 km in margin areas of the Pacific Ocean basin to 60 km in the Tien Shan. Average crustal thicknesses within the platforms are in the region of 40 km, increasing to 50 km in the southern outlying areas of the East-European platform and to 46 km in areas of recent tectonic activity within the Turanian and Siberian Platforms.(4) In deep depressions, internal and outlying seas, the crustal thickness is reduced, and 'granite' layer (6.0–6.5 km/s) is absent. Mesozoic formations of the Far East and adjacent zones of transition from the Pacific Ocean to the continent are distinguished by reduced thicknesses. In Alpine formations the 'basalt' layer dominates over the 'granite' layer.(5) Subhorizontal crustal stratification complicated by deep faults has been found everywhere. Plicate geosynclinal folding is not reflected in deep crustal structures.(6) Temperature of the platform folded basement is mainly related to thickness and composition of the sediments.The transcontinental sections provide a clearer idea of the relationship between anomalous geophysical fields and deep and shallow crustal structures than is given in corresponding maps.