tectonic cycle[tek′tän·ik ′sī·kəl]
(or orogenic cycle), a period of geologic history that is characterized by a certain sequence of tectonic and other geologic events. Such periods last for more than 100 million years.
Tectonic cycles are evidenced most clearly in geosynclinal regions, where the cycle begins with the subsidence of the crust and the formation of deep marine basins. The first phase is characterized by the accumulation of thick layers of sediments, by submarine volcanism, and by the formation of basic and ultrabasic intrusive magmatic rocks. Subsequently, island arcs emerge, andesitic volcanism occurs, the marine basin is broken up into smaller basins, and fold-overthrust deformations begin. In the next phase, folded and nappe mountain structures form that are surrounded and divided by marginal and piedmont foredeeps and intermontane troughs. These foredeeps and troughs are filled with molasse—that is, products of the wearing down of the mountains. Also characteristic of this phase are regional meta-morphism, the formation of granite, and rhyolitic and basaltic terrestrial volcanic outpourings.
A similar sequence of events is observed in cratons. Continental conditions give way to a marine transgression. Subsequently, a regression occurs, and a continental regime is established with the formation of a weathering mantle. The cycle is accompanied by a corresponding variation in sediment types. Continental sediments are followed by lagoonal sediments, which are often sa-liferous or coal bearing. Marine detrital sediments are the next type encountered. The middle of the cycle is characterized primarily by carbonaceous or siliceous sediments. At the end of the cycle marine, lagoonal (saltine), and continental (sometimes glacial) sediments are again found.
Intensive fold-overthrust deformations and mountain formation in some geosynclinal zones may be associated with the formation of new, adjacent zones of geosynclinal structure and with the formation of rift systems, called aulacogens, in the cratons.
The average duration of tectonic cycles in the Phanerozoic has been 150–180 million years. The cycles were apparently longer in the Precambrian.
In addition to the tectonic cycles described, still larger cycles are sometimes identified. They are known as megacycles and last hundreds of millions of years. In Europe, and partially in North America and Asia, the following cycles have been established in the late Precambrian and Phanerozoic: Grenville (middle Riphean), Baikalian (late Riphean-Vendian), Caledonian (Cambrian-Devonian), Hercynian (Devonian-Permian), Cimmerian (Trias-sic-Jurassic), and Alpine (Jurassic-Cretaceous-Cenozoic).
The original schematic notion of tectonic cycles has been subjected to considerable well-founded criticism. In this conception, tectonic cycles are rigidly synchronous on a planetary scale, recur throughout the world, and are characterized by the same complex of phenomena. In reality, the end of one cycle and the beginning of other cycles often occur simultaneously (in different but often adjacent regions). Moreover, in each individual geosynclinal system the one or two cycles immediately preceding its conversion into a folded mountain system are usually the most fully expressed cycles. The earlier cycles do not exhibit all the typical phenomena and sometimes merge into one another. On the scale of the entire history of the earth, the tectonic cycle pattern represents just a complication of the planet’s general development. The individual cycles constitute phases of the megacycles, and the megacycles form the large phases of earth history as a whole. The causes of this cyclicity have not yet been established, although various hypotheses have been advanced. One hypothesis, for example, posits a periodic accumulation of heat and a periodic increase in the heat flow from the deep interior of the earth. Another hypothesis suggests the existence of cycles of upwelling or circulation (convection) of the products of differentiation of mantle material.
V. E. KHAIN