Silence cycle

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The Stille cycle is a geological term . Around 1920 Hans Stille formulated his magmatic-tectonic cycle based on the similarities in the structure of the different ancient European mountains . The Stille cycle remained valid until the 1960s, but has now been overtaken by the theory of plate tectonics . Nevertheless, it is still useful today when it comes to merely describing the development of a piece of continental crust. Plate tectonics, on the other hand, provides the causes or the mechanism for the tectonic movements.

The stages of the silence cycle

The four stages of a Stille cycle are:

tectonic stage igneous stage
4. Cratonic final basaltic volcanism
3. quasicratonic Subsequent acidic to basic volcanism and plutonism
2. orogenic synorogenic granitic plutonism
1. synclinal initial basic volcanism

Plate tectonic interpretation

After the theory of plate tectonics has been developed, the individual phases of the Stille cycle can be viewed in the light of more recent findings.

Syncline phase

A Stille cycle begins with the subsidence of a rift system (a rift) within the continental crust (the process is known as taphrogenesis or English rifting ). The Oberrheingraben is an example of a (albeit stuck) taphrogenesis.

Concomitant phenomena at this stage are coarse-plastic sediment series and often evaporite formations (rocks such as salt or anhydrite caused by chemical precipitation ) and volcanic rocks. If the expansion movements continue, the continental crust tears and an initially narrow ocean basin, a geosyncline, is created . The basic volcanism is represented by the newly formed oceanic crust and the volcanic rocks of taphrogenesis.

As a result of subsidence movements ( oceanic crust is denser than continental), the basin initially absorbs massive series of sediments; under appropriate climatic conditions, carbonate platforms may form . With continued expansion, a central ocean forms with thin pelitic sediment cover . Further formation of sediment wedges takes place at the passive continental margins .

Orogenous phase

The extension movement is finally reversed and the two continental margins begin to approach each other again. At this stage, oceanic crust subduction occurs . The subduction can take place either within the oceanic area or on one of the continental margins.

In the first case a volcanic island arc is created , in the second an active continental margin . At the front of the subducting plate, deformations and the formation of accretion wedges occur (in these there are simultaneously deposited layers of sediment in multiple succession, as if planed from the subducted plate). Further in the direction of the actual subduction it comes under the upper continental plate to the melting of the subducting plate and to the emergence of silicon-rich (acidic) to intermediate plutonic rocks such as granites , diorite or tonalite , and corresponding volcanic rocks , such as dacites , rhyolite or andesite ). The actual orogenic (mountain-forming) stage only occurs when the two continental margins collide. This is where the strongest deformations take place, in some cases too strong horizontal displacements , since the two continental margins usually do not match. Usually only an area with relics of oceanic crust remains of the ocean, a so-called suture . Oceanic crust autopsies are rare .

Subsequent phase

The quasi-cyclonic stage begins with the end of the compressive movements with gravitational fracture tectonics . This may also lead to the loss of sliding surfaces near the surface. The associated subsequent volcanism and plutonism is similar to that of the orogenic stage. From the point of view of plate tectonics, the quasi-cyclonic stage cannot be completely delimited.

Cratonic phase

If the tectonic calm continues, the relief will be leveled and thus the cratonic stage. However, the consolidation and the associated basaltic volcanism in the sense of Stilles is no longer tenable today, because Stille assumed that the ocean formed in the synclinal stage goes into orogenesis as a whole, folded. Rather, there is an almost complete destruction (subduction) of the oceanic crust, at most residues of ophiolite complexes remain in the suture zone.

literature

See also