median mass[′mē·dē·ən ′mas]
a relatively stable block of the earth’s crust, more or less isometric in outline, located within a geosynclinal belt (region) and separating geosynclinal systems. Median masses differ from geosynclinal systems both in their lower mobility and in the development of the ancient, primarily Precambrian, continental crust forming the basement of the mass.
Median masses are apparently relict fragments of the ancient continental foundation on which the intercontinental geosynclinal belts of the Phanerozoic eon occurred. The accumulation of thick geosynclinal strata (formations) in adjacent geosynclinal systems on median masses corresponds to the formation of a thinner sedimentary cover composed of shallow-water marine and continental sediments and volcanic rocks. In certain cases, narrow geosynclinal troughs, distinguished by the short time required for their development, may intrude into the body of median masses along faults. The sedimentary, volcanogenic cover of the median mass is ordinarily subjected to less intense deformation than the strata of geosynclinal systems; like faults, these deformations are unevenly distributed and often have irregular trends. The magmatism of median masses is seen primarily along the periphery (volcanic-plutonic belts), and the products of magmatism are usually distinguished from geosynclinal magmatic rocks by a higher silica and/or alkali content.
In the orogenic phases of development of geosynclinal belts, median masses become involved in either the subsidence of inter-montane troughs with accumulation of molasse or the uplifting of adjacent fold mountain structures. Uplifted median masses are often subjected to intense granitization, and the thickness of their crust is significantly increased; such median masses are frequently thrust over adjacent geosynclinal systems, tectonically covering the internal zones of the latter. By contrast, the peripheral parts of subsided median masses are often covered by superimposed folded strata of adjacent mountain structures. At the end of separate tectonic phases, certain median masses expand at the expense of neighboring geosynclinal systems owing to folding, me-tamorphism, and granitization.
Examples of median masses are the Rhodope Mountains on the Balkan Peninsula, the Kokchetav-Muiunkum Massif in the (Paleozoic) region of Central Kazakhstan and northern Tien-Shan, and the Indochinese Central Massif in the Paleozoic-Meso-zoic folded geosynclinal region of Southeast Asia.
V. E. KHAIN