Extension (geology)

Extension describes tectonic processes that are related to the expansion of the earth's crust or lithosphere . This is also called expansion tectonics . Geological structures created by extension or expansion tectonics are referred to as extensional or extensive structures or expansion structures.

Zones with extensional tectonics

Zones with extensional tectonics are typically associated with:

Horst-and-ditch structures, as they typically arise from large-scale expansion of the crust (the direction of extension is indicated by the red arrows).

Continental rifts

Rifts are linear zones with local crustal expansion. Their size ranges from less than 100 km to several hundred kilometers, consisting of one or more normal faults and the associated blocks. In a single rift segment, one polarity (i.e. line of incidence) usually dominates, the result of which is a half-ditch geometry. Other common geometries are metamorphic core complexes and tipping clods. Examples of active continental rifts are the Baikal Rift and the East African Rift .

Divergent plate boundaries

Divergent plate boundaries are zones with active expansion tectonics, in which the earth's crust formed by mid-Ozan ridges is included in the opening process.

Gravitational thinning of thickened earth's crust

Zones with thickened crust, such as those created by continent-continent collisions, tend to spread laterally; this spreading also occurs during the collision process. After the end of the collision, the thickened crust undergoes a post-orogenic gravitational collapse, which often results in very large elongation disorders. The large-scale Devonian expansion, for example, followed immediately after the end of the Caledonian orogeny , particularly in East Greenland and western Norway.

Relief zones along blade shifts

When a leaf displacement is shifted along the stroke, zones of stretch or overstretch are formed, which can also form gaps. Such areas are also known as relief zones and often form pull apart pools. Examples of active pull apart basins are the Dead Sea , which was created by the sinistral displacement of the Dead Sea Transform fault system, and the Sea of Marmara , which was created by the dextral displacement of the North Anatolian fault .

Backarc basin

Backarc basins are formed behind several subduction zones as a result of the slap rollback expansion zones parallel to the island arches.

Passive continental borders

A passive continental margin, formed over a weaker layer, such as over-compressed claystone or rock salt , tends to expand laterally under its own weight. The inner part of the sedimentary triangle is influenced by spreading disturbances, which is balanced by shortening at the outer edge.