Tectonic Cycles and Tectonic Timescale

The subdivision of the tectonic or geological cycle affords a basis for research on geotectonic evolution. However, since the emergence of plate tectonics, some scholars who adopt the philosophy of uniformitarianism have tried to abandon the concept of tectonic cycle. Nevertheless, with the elapse of time, the appearance of earth system science, and the observations based on large-scale and multidisciplinary geoscience, people have got to know the importance of sudden and catastrophic changes in the course of crustal evolution, and recognized that the idea of cyclic evolution arguing that the structure of the earth crust develops cyclically and in a spiral-like manner with gradual and sudden changes in combination with each other is a powerful weapon for more comprehensive and profound understanding of the geological laws. In the study of geotectonics, a number of geologists are inclined to substitute the stratigraphic timescale for the tectonic cycle. However, the Phanerozoic chronostratigraphic chart framed according to biological stratigraphy does not fully agree with tectonic cycle and tectonic-magmatic event. This is because the chronostratigraphic chart is established on the basis of the study of the earth’s exogenesis characterized by the interaction between the lithosphere, hydrosphere, atmosphere and biosphere, whereas the tectonic cycle is the historical record of earth’s endogenesis characterized by the multisphere interaction of the crust, mantle and core, as well as different layers within them. Some scholars have used tectonic (tectonic-magmatic) events represented by isotopic ages instead of tectonic cycles in studying the geotectonics. However, the “event” is just a display of a single phenomenon, and the “cycle” is the illumination of process, which reflects the internal links between “events” and the essence of the evolution. In fact, the establishing of the Wilson cycle has bestowed new scientific connotations on the connections between tectonic cycles and tectonic events, which can serve as a shining example of handling the relationship between inheritance and innovation properly. Doubting about the synchroneity of the global orogeny, some scholars consider that it is impossible and unnecessary to establish a unified global tectonic timescale. Nevertheless, the dynamic activities of the earth as a whole should be substantially simultaneous. Within a mega-region under the control of a unified geodynamic system, the tectonic movements did not take place exactly at the same time in different places, but they were roughly simultaneous. The tectonic movements of the Caledonian and Variscan cycles in North America, Europe and Asia are basically comparable with each other, which can serve as a convincing proof. It is therefore held that, in accordance with the principle of priority, the early and late Paleozoic cycles can be called the Caledonian and Variscan (Hercynian) cycles, respectively. The emergence of the theory of the supercontinent and supercontinent cycle has not only deepened the study of tectonic cycles, but also paved the way for establishing tectonic timescale. Some proposed supercontinent cycles, such as the Paleoproterozoic Columbia, Mesoproterozoic Rodinia, Neoproterozoic Gondwana and the Phanerozoic Pangea, could be used as the first-order time unit for the tectonic timescale. Each supercontinent cycle can be further divided into several cycles, such as the Pangea with the subdivision of the Caledonian and Variscan cycles. It is believed that, like the Phanerozoic chronostratigraphic chart, the mega-regional or even global tectonic timescale will be established in the near future with the deepening of geological, geochemical and geophysical research and the more comprehensive and systematic observation of the solid earth system.