Elsevier

Tectonophysics

Volume 146, Issues 1–4, 30 January 1988, Pages 1-45
Tectonophysics

Earth structural patterns and rhythmic tectonism

https://doi.org/10.1016/0040-1951(88)90079-0 Get rights and content

Abstract

An analysis carried out across the globe shows that the surface is characterized by major megalineaments (corresponding to megashear zones) which are arranged according to intersecting planes lying on great circles of the Earth. Two diagonal and conjugate systems of great circle shearing planes emerge, one low-angle and synthetic (dextral) and another high-angle and antithetic (sinistral) relative to the present equatorial belt. Such fundamental orthogonal lineaments are represented by the two Meso-Cenozoic mountain belts, the modern outline of the continental margins and the coastlines, the mid-ocean ridges and related fracture zones, the intracontinental rifts, the river courses and other linear morphotectonic structures. Some old mountain ranges, young rift zones and young oceanic ridges lie along the same structural great circle plane, showing therefore a common origin related to activation of deep geofracture zones in the basement. Particularly evident is the link between the Paleozoic Appalachians, the young East Pacific Rise, the intermediate Pacific, the Antarctic and the Southeast Indian rises (Fig. 1). Apart from this, it seems that the characteristic sinuosity of the Pacific and Indian rises is merely the result of the cartographic distortion of a great circle megashear structure due to map projections. Instead, the mid-Atlantic Ridge seems to be made up of several segments, alternatively representing parts of the two conjugate shear planes. The link between continental and oceanic features and the similarity of their tectonic and magmatic segmentation seems to indicate that they represent different stages of a developmental sequence displayed in space, which leads to the transformation of a continental area into an oceanic one (i.e., “oceanization”). This evolutionary sequence might comprise: (1) elevated young mountains, (2) Basin-and-Range-type structure, (3) small ocean basins, (4) Pacific-type rise systems and (5) Atlantic-type ridge systems. This also implies a strike-slip nature and torsion deformation not only of the basement fractures within the orogenic belts, but also of the mid-ocean ridges and small ocean basins.

Examination of intracontinental rifts, such as the East African and the Baikal rifts, indicates a repetition of the diastrophic episodes in time due to tectonic rejuvenation and reactivation of ancient geofractures of the basement due to high heat flow. From the study of the main mobile belts, such as the orogenic and continental margin zones, it emerges that we are not dealing with a pure and simple repetition of similar geological events, but rather with a space-time unidirectional cyclicity of a spiral-type. The whole stratigraphie column is basically composed of episodes of tectonism, magmatism and sedimentation, which correlate with the sea level fluctuations of the classical geotectonic cycle. The transgressive hemicycle is believed to be determined by two different types of crustal movements; the krikogenesis type which creates the ridge systems and the opening of the shear basins and the geosynclinal type which creates (within such long shear basins) regional subsidence with deep root formation (i.e., the “cyclonic-like vortices” inside the so-called subduction zones) (Fig. 4). In its turn, the regressive hemicycle might be due to tectogenesis (i.e., strike-slip compression, closure and suturing of the geosynclinal belts with folding and thrusting and formation of the orogens) and to epeirogenesis (i.e., the vertical tectonic elevation of the orogenic belts, creating high mountain ranges). The remarkable passage from horizontal crustal contraction (tectogenesis) to rapid vertical uplift (epeirogenesis), is considered to be a global phenomenon which is produced by a fundamental geodynamic change which seems to be controlled by a major rotational change of the Earth.

In fact, a new global working hypothesis, named helicyclic tectonics, is being set out here, according to which, the whole of the interrelated tectonic, magmatic, depositional, climatic, biotic and magnetic phenomena is basically a manifestation of the variations (cyclic in character) in the Earth's rate of rotation during the course of geological time. From the beginning of the geotectonic cycle up to the compressional tectogenesis which creates the orogens, there is a long period of acceleration in rotation, accompanied by a pulsating increase of the Earth's polar flattening and an increase in its equatorial bulge. Thus, approximately N-S compressive stresses in the crust are generated, which again reactivate the ancestral geofractures with horizontal shearing movements capable of slicing away slivers from the continental margins. These are successively and partially “oceanized” by flood basalts (“megabreccia”, see Fig. 2) and partially transported by strike-slip faulting in other latitudes and then rewelded to form strike-slip orogens. During the climax of tectogenic compression, the maximum is reached in the spin rate and in the polar flattening. There follows a period of deceleration in rotation during which the shape of the geoid passes from an oblate to a more prolate-shaped configuration, due to a decrease in the equatorial bulge. A global regression, active emergence of mountains and a geocratic phase of maximum continentality in the low latitudes follows this. At the end of the cycle, the minimum in the rotation rate is reached. The worldwide (pangeodic) cyclic cadence in the whole series of geological, climatic and biological phenomena is considered as a second-order manifestation of what can be perhaps described as “the breathing Earth”.

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