Under grave fraction (also rift , Rift Valley of Engl. Rift : crack, column) is understood in the geology an elongated tectonic elongation zone, at a relatively narrow range crust deep into the crust reaching along dislocations lowers. The faults may already have existed, but could also have arisen again due to the tensile forces. Active rift breaks are also sedimentary basins .
Rift fractures occur when the continental crust is stretched by various causes. Like any relatively brittle material, the crust can break and give way under this tensile stress (see also trench (geology) ).
In the case of continental rift zones, based on the dynamic processes that lead to their formation, a distinction can be made between passive rifts and active rifts .
In the case of a passive rift , a crustal area reacts to tectonic forces, which are usually caused by plate tectonic processes in the vicinity, e.g. B. a mountain formation , go out. One hypothesis is based on the assumption that the Upper Rhine Rift Valley and other related rift breaks in Central and Western Europe were created as a result of east-west stretching across the northward compression of the Europe-Africa convergence.
Active Rifts emerge over Manteldiapiren ( ger .: mantle plumes ) in so-called hot spots thinned lithosphere. Here, the flow of hot jacket material at the hot spot causes the lithospheric plate to be heated from below, which means that it is thermally raised over a large area, slowly melted and thinned out, e.g. B. at the East African Trench .
Both active and passive rifts usually lead to the formation of relatively narrow trench systems with high trench shoulders. Typical of the grave structures are relatively flat earthquake centers and often extensive, bimodal volcanism, i.e. both basic and rock volcanism with shield and crevice volcanoes . Rift fractures are the only large tectonic milieu in which carbonatitic volcanism occurs, e.g. B. in the case of the Kaiserstuhl or the Ol Doinyo Lengai . As the crust subsides, the center of the rift can sink below sea level and then be flooded by the sea. So find z. B. in the Upper Rhine Graben and the associated Mainz Basin, fossil-rich marine deposits from the Oligocene and Miocene . If the rift zone lies far in the interior of the continent, a lake or a lake landscape will develop there.
With ongoing active rifting , the expansion of the crust can no longer be compensated for by the sinking of continental crustal clods and the rift volcanism produces new, basaltic (oceanic) earth crust along the trench axis. A continental rifting turns into a mid-ocean ridge between two divergent tectonic plates .
Lithosphere, which has been thickened, heated up and disrupted by mountain formation, tends to give way under its own weight as soon as the tectonic compressive pressure is gone. This creates basin and ditch structures in the interior of the mountains, such as B. Today in the so-called Basin-and-Range-Province in the southern Rocky Mountains . Something similar happened in the Upper Carboniferous and the Permian , about 300 million years ago in the Variscan Mountains in Central Europe.
Examples of rift zones on earth
Active rifts (in the sense of triggered by mantle diapirs )
- The Great African grave breach ( Great Rift Valley ) stretches from Lebanon to Mozambique and is caused by the drifting apart of the African , the Arabian Plate and the Somali Plate .
- The East African Rift is part of the Great Rift Valley . Between the African and the nascent Somali plate, expansion tectonics has led to increased volcanism along the rift zone over a length of 5000 km and to a partial sinking of the rifts below sea level. In the next 10 to 20 million years, the Red Sea can be expected to penetrate this rift valley and East and West Africa to drift further apart.
- The Baikal Trench in southern Siberia . In the case of the Baikal Trench, it is alternatively assumed that it was formed as a passive rift by the Amur Plate moving eastwards as a result of the collision of the Indian and Eurasian plates .
- The Tschersky Rift , in northeast Siberia, between the Eurasian and North American plates (the mid-ocean ridge of the Arctic Ocean extended to the continent).
- The Upper Rhine Rift Valley in southwest Germany , bordered by the Vosges and Palatinate Forest in the west and the Black Forest and Odenwald in the east.
- The Elbe zone between the Erzgebirge Pultscholle in the southwest and the Lausitzer Platte in the northeast
- The Egergraben between the Erzgebirgscholle in the northwest and the Bohemian Massif (including the Kaiserwald ) in the southeast
- The Oslograben in southern Norway, in the southern part of which the Oslofjord is located. This trench structure has been inactive for several hundred million years.
- Here, between "active" in the sense of tectonic movements taking place at the fault zones of the rift structure at the time under consideration (opposite: inactive rift) and "active" in the sense of active rifting = rifting as a result of mantle diapirism (opposite: passive rift ) to distinguish!
- Christiane Martin, Manfred Eiblmaier (Hrsg.): Lexicon of Geosciences: in six volumes, Heidelberg [among others]: Spektrum, Akad. Verl., 2000–2002
- Peter A. Ziegler: European Cenozoic rift system. Tectonophysics. Vol. 208, No. 1-3, 1992, pp. 91-111, doi: 10.1016 / 0040-1951 (92) 90338-7 .