An increase in the Earth's rotational velocity apparently commenced in the first half of the Carboniferous period, and its prevalence led to the formation of the folded structures of the Urals in the second half of the Carboniferous period and of the Appalachian folded structures in the Upper Carboniferous—Lcwer Permian period.
This was apparently again followed, in the Permian period, by a decelera tion of the Earth's rotation which led to a tensile deformation of the crust at the end of the Permian and the beginning of the Triassic in the polar regions, on the Siberian platform, accompanied with faults and profuse fissure eruptions of basic magma (the Siberian traps). The tensile deforma tion of the Earth's crust continued throughout the Triassic, Jurassic, and Upper Cretaceous periods, involving various areas in the polar region. Its traces have been preserved in the extensive fault zones which are interlaced by dikes of porphyrites and porphyries, as well as in fields of basalt and andesite eruptions in northeastern USSR, in the Cordilleras, in the Rocky Mountains, and elsewhere. The tremendous scale of the tensile crustal deformation in northeastern USSR is evidenced by the large troughs, the numerous steeply dipping and fairly thick dikes of porphyrites and porphyries occurring over very extensive areas, the long, narrow belts of effusives (traces of fissure eruptions), the nonuniform radial uplifts of individual crustal fragments, etc.
Acceleration of the Earth's rotation was apparently again prevalent at the beginning of the Upper Cretaceous period, resulting in the formation of folded structures in northeastern USSR, in the Cordilleras, and in the Rockies, and to the formation of tensile deformations and faults of the Earth's crust in the equatorial region — in Africa, and India, in the northern Indian Ocean, and probably also in South America. These tensile deformations and faults of the crust were accompanied with intrusions of basic magma into numerous fissures (e.g., in Africa) and with large scale fissure eruptions of basic magma to the surface (e. g., in India, and in the Indian Ocean). In the Cordilleras and the Rockies the folding continued into the Lower Tertiary period.
The prevailing deceleration of the Earth's rotation apparently recurred in the second half of the Tertiary, and has continued to the present time.
It generated tensile deformation and faults of the Earth's crust in numerous areas in the polar regions as well as large-scale, mainly fissures eruptions of basic and intermediate magma. Huge fault fissures which served as outlets for the eruption of thick basalt sheets are known in Greenland, Iceland, the northern Atlantic, Bolshezemel'skaya Tundra, Spitsbergen, eastern Transbaikalia, Mongolia, northeastern USSR, Alaska, British Columbia, and other regions. No true folded formations (true crustal folding) on any considerable scale occurred during this period in the polar regions. According to geological observations, the orogenesis in Alaska, the Rockies, and other areas during this period took the form of nonuniform (radial) uplifting of individual crustal blocks between faults, most probably as a result of the tensile deformation of the crust combined with its radial uplifting.
The period between the formation of the folded structures of the Urals and those in northeastern USSR, the Appalachians, and the Rockies lasted approximately 175 to 180 million years. Indeed, the commencement of the most intensive folding processes in the Urals is dated as the end of the Middle and the beginning of the Upper Carboniferous period, while the commencement of the most intensive folding processes in northeastern USSR is dated as the end of the Lower Cretaceous and the beginning of the Upper Cretaceous period. According to the geochronological scale developed by Holmes and Kay (1951), the boundary between the Middle and Upper Carboniferous period lies approximately 265 to 270 million years ago, and approximately 285 to 290 million years ago according to the scale developed by Polevaya (1961); the boundary between the Lower and the Upper Cretaceous period is dated as approximately 90 to 100 million years ago on the former scale, and as approximately 105 to 110 million years on the latter. The period intervening between the folding periods was, therefore, about 175 to 180 million years.
A period of approximately the same duration intervened between the formation of the largest crustal fault zones which was accompanied with large-scale magma eruptions (mainly basic) and the nonuniform uplifting of crustal fragments in the Upper Tertiary--Quaternary and in the Permian—Triassic—Jurassic. Early crustal faults with profuse eruptions of basalts are known in the Lower Eocene of North America (in Washington and Oregon) and eruptions of andesites are known in the Absaroka Mountains in the Rockies and on the Yellowstone Plateau. According to the geochrono logical scale, these eruptions occurred approximately 40 to 50 million years ago. Early tensile deformations of the crust and fissure eruptions of basic magma during the Early Mesozoic period of deceleration of the Earth's rotation had already commenced by the Upper Permian period, approximately 230 to 220 million years ago on Polevaya's geochronological scale. The intervening period was, therefore, again approximately 180 or 170 million years. This interval between successive periods of compressive deformations of the Earth's crust and those of tensile deformations is very similar to the so-called anomalistic period of the revolution of the solar system about the central galactic masses, determined to be approximately 176 million years by Parenago (1952). This fairly close agreement can hardly be accidental. Evidently, variations in the Earth's rotational velocity are closely related to the nonuniformity of this galactic movement of the solar system.