a linear depression of the earth’s crust that is formed by tectonic movements. Tectonic troughs exhibit great diversity because they result from the action of different forces at different stages in the evolution of the crust. A tectonic trough is usually associated with a tectonic uplift of the same age.
Several types of tectonic troughs can be distinguished. Geosynclinal tectonic troughs, for example, are narrow zones of deep subsidence. The zones fill with thick strata of volcanic and sedimentary rocks that subsequently undergo folding. These troughs usually occur under conditions of intense extension of the earth’s crust. Long trench-shaped geosynclinal troughs are sometimes called tectonic trogi in Russian (N. S. Shatskii, 1946). Large syn-clinoria usually form at the site of such troughs during folding. Geosynclinal tectonic troughs termed eugeosynclinal and located on the oceanic side of folded regions away from the craton develop on oceanic-type crust. Examples are the Magnitogorsk and Tagil troughs in the Urals. Geosynclinal tectonic troughs termed miogeosynclinal and located on the cratonic side of folded regions are underlain by continental crust.
Another type of tectonic trough is the foredeep. Foredeeps are formed in ages of folding and mountain building on the margins of cratons under conditions of compression before the fronts of growing mountain chains. Examples are the Cisuralic, Prealpine, and Mesopotamian troughs. Foredeeps fill with the products (primarily molasse) of the wearing away of the adjacent mountains. The subsidence of foredeeps compensates to a considerable degree for the uplift of the adjacent folded structures.
Intermontane tectonic troughs arise in the rear of folded regions during the orogenic stage of geosynclinal development. Such troughs are tectonic basins between mountain uplifts (ranges). Like foredeeps, intermontane troughs fill with molasse, which is often associated with manifestations of land volcanism, as in the Minusinsk and Tuva intermontane troughs of the Altai-Saian region.
Among intracratonic tectonic troughs, a special place is occupied by rift valleys (rift troughs), which may be ancient or young. Ancient rift valleys, or aulacogens, are large graben-like structures within cratons; an example is the Pachelma trough. Young rift valleys are found, for example, in the East African rift system. Rift valleys are characterized by layers of fresh-water and continental sediment; volcanic outpourings, particularly basalts, are common. Intracratonic tectonic troughs reflect the splitting of the continental foundation of the cratons.
Submontane tectonic troughs (predgornye tektonicheskie progiby), such as the Irkutsk trough, arise during epicratonic orogenies. They fill with molasse-type deposits without the participation of volcanism.
Tectonic troughs are divided into inherited and superimposed types. Inherited troughs occur without a major discontinuity at the site of regions of earlier downwarping. Superimposed troughs are troughs that are newly formed, after a substantial interval, on various more ancient structures, which they cut across unconformably. Tectonic troughs have the form of gently sloping synclines, grabens, and synclinoria; in many cases, the sedimentary strata of tectonic troughs are displaced laterally by pressure to form overthrust nappes. Examples of contemporary tectonic troughs are deep-sea trenches, the narrow interarc troughs separating chains of island arcs, certain troughs of marginal and inland seas, and the rift valleys of the Red Sea.
REFERENCEKhain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
L. P. ZONENSHAIN