Crustal shear patterns and orogenesis

Abstract

Major crustal shears of North America, Europe and Africa are shown and analyzed; and it is concluded that two orthogonal compressional shear sets, which are essentially wrench-fault zones, exist world-wide. These sets are thought to have been generated by meridional and equatorial compressive stresses; the major elements of the meridional and equatorial shear systems for the world are shown. Lateral compression in the earth's crust is thought to be the cause of these major shears and the resulting crustal fragmentation into polygonal blocks. Movement on the major shears localizes increases of temperature and pressure along the shear zones — the result is mobilization of crustal material along the edges of the crustal blocks. Mobilization of crustal material along the major shear zones results in one or more of the five types of tectonic manifestation described below, as well as related magmatism and metamorphism, and is the underlying cause of orogenesis. Orogenesis is seen to be a natural consequence of the fragmentation of the earth's crust under lateral compression, and results from the interaction of the crustal blocks as they move and yield in response to the lateral-compression stress field and the earth's gravitational field. On this basis tectonic mountains are classified into: 1. (a) linear uplifts with longitudinal wrench-fault zones and related thrusting; 2. (b) autochthonous fold belts; 3. (c) vertically uplifted or tilted fault blocks; 4. (d) domal uplifts; and 5. (e) volcanic chains. Secondary effects of orogenesis include metamorphism and magmatic activity related to frictional heat from movement in major shear zones; and erośión, glaciation and gravity sliding resulting from vertical components of movement along major faults. Furthermore, orogenesis is the principal process by means of which “continentization” and “oceanization” take place, which result in crustal differentiation into granitic and basaltic elements. Continental drift may well be merely movement of polygonal crustal blocks, or groups of blocks, along the major shear zones. It is thought that continental drift occurs by translation (with very little, if any, rotation) of the polygonal crustal blocks, which derive from the regmatic shear pattern, moving above a discontinuity which may be the Mohorovičić discontinuity at the base of the crust, or may be deeper. The result is a conceptual earth model wherein a relatively thin crust overlies a more mobile sub-crust which is fundamentally basaltic in composition, as suggested by Cotta (1858). The crust is fragmented into polygonal blocks by major wrench faults of the regmatic shear pattern. Interaction of the crustal blocks under ubiquitous lateral compression results in orogenesis, and concomitant “oceanization” and “continentization” of the crust. The crustal blocks, of either oceanic or continental composition, can move relative to the sub-crust by translation over considerable distances (1,200 km or more). It is hoped that these considerations may help us to come to a better understanding of the structure of our earth, which, even with all its disconcerting complexities and its fascinating similarities, should be understandable if viewed as a whole.