Orogeny, geosyncline and continental drift

Abstract

Most of the existing theories agree that mountains are formed in geosynclines. But a eugeosyncline is not, contrary to common belief, a long, narrow, constantly deepening, single basin. It is divided into smaller basins and emergent areas, called intra-geosynclines and intra-geanticlines (Beloussov, 1962). What probably happened was that at the end of a long period of “geosynclinisation” the forces responsible for this activity either shifted or simply ceased. The intra-geanticlines were then already up-warping, the intra-geosynclines started upwarping isostatically. Such epeirogenic upwarping is a very slow process. At best it could result in small isolated peaks, and not in mountain ranges. These isolated peaks could not have attained the heights reached by the Himalayan peaks. It is also pointed out that several ranges in the Tertiary mountain belts, e.g., the Himalayas, the Alps, the Rockies, and the Andes, were formed without geosynclines. Conversely, in the Gulf Coast “geosyncline” an immense thickness of undeformed sediments has accumulated and there is no reason to believe that this will give birth to a mountain range. Thus, it appears that extensive and highly folded ranges can come into being without a geosyncline existing in the area, whereas exceedingly thick sedimentary prisms need not lead to mountain formation. The current concepts are, therefore, in need of revision. Continental drift, on the other hand, appears to explain the phenomenon satisfactorily. Thus, the Tertiary mountain ranges may be divided into three broad categories: 1. (1) Those in which two drifting crustal blocks collided and the sediments in between have been crumpled into a mountain chain. 2. (2) Those in which a drifting crustal block has been resisted by the ocean floor. 3. (3) Those in which an orocline has resulted in extensive block faulting and horst type of mountain on the obtuse side of the “fulcrum”. A geosyncline is essential for none of these and it is contended that the Tertiary mountains, by and large, were formed without the existence of a geosyncline, and were, perhaps, a result of continental drift. Therefore, it follows that unless drift had taken place earlier, there need not have been any mountain ranges on the earth. The intense folding and faulting seen in former geosynclinal belts must, then, be the result of oscillatory movements in the small basins within the geosyncline. From their similarity in faults and folds to the conditions in the existing mountain ranges it need not be concluded that mountains were formed only in former geosynclines.