Editing Sedimentary basin (section)

== Mechanics of formation ==
Sedimentary basins form as a result of regional subsidence of the lithosphere, mostly as a result of a few geodynamic processes.

=== Lithospheric stretching ===
[[File:Formation of passive margins cropped.svg|thumb|right|Illustration of lithospheric stretching]]
If the [[lithosphere]] is caused to stretch horizontally, by mechanisms such as rifting (which is associated with divergent plate boundaries) or ''ridge-push'' or ''trench-pull'' (associated with convergent boundaries), the effect is believed to be twofold. The lower, hotter part of the lithosphere will "flow" slowly away from the main area being stretched, whilst the upper, cooler and more brittle [[Crust (geology)|crust]] will tend to fault (crack) and fracture. The combined effect of these two mechanisms is for Earth's surface in the area of extension to subside, creating a geographical depression which is then often infilled with water and/or sediments. (An analogy is a piece of rubber, which thins in the middle when stretched.)

An example of a basin caused by lithospheric stretching is the [[North Sea]] – also an important location for significant [[hydrocarbon]] reserves. Another such feature is the [[Basin and Range Province]] which covers most of Nevada, forming a series of [[Horst (geology)|horst]] and [[graben]] structures.

Tectonic extension at divergent boundaries where continental rifting is occurring can create a nascent ocean basin leading to either an ocean or the [[aulacogen|failure of the rift zone]]. Another expression of lithospheric stretching results in the formation of ocean basins with central ridges. The [[Red Sea]] is in fact an incipient ocean, in a plate tectonic context. The mouth of the Red Sea is also a tectonic [[triple junction]] where the Indian Ocean Ridge, Red Sea Rift and [[East African Rift]] meet. This is the only place on the planet where such a triple junction in oceanic crust is exposed [[subaerial]]ly. This is due to a high thermal buoyancy ([[thermal subsidence]]) of the junction, and also to a local crumpled zone of seafloor crust acting as a dam against the Red Sea.

=== Lithospheric flexure === 
[[File:Viscoelastic lithospheric flexure.png|thumb|Schematic illustration of viscoelastic lithospheric flexure]]
[[Lithospheric flexure]] is another geodynamic mechanism that can cause regional subsidence resulting in the creation of a sedimentary basin. If a load is placed on the lithosphere, it will tend to flex in the manner of an elastic plate. The magnitude of the lithospheric flexure is a function of the imposed load and the ''[[flexural rigidity]]'' of the lithosphere, and the wavelength of flexure is a function of flexural rigidity of the lithospheric plate. Flexural rigidity is in itself, a function of the lithospheric mineral composition, thermal regime, and effective elastic thickness of the lithosphere.<ref name="AllenandAllen" />

Plate tectonic processes that can create sufficient loads on the lithosphere to induce basin-forming processes include:
* formation of new mountain belts through [[orogeny]] create massive regional topographic highs that impose loads on the lithosphere and can result in [[foreland basin]]s.
* growth of a [[volcanic arc]] as the result of subduction or even the formation of a [[Hotspot (geology)|hotspot volcanic chain]].
* growth of an [[accretionary wedge]] and thrusting of it onto the overriding tectonic plate can contribute to the formation of forearc basins.

After any kind of sedimentary basin has begun to form, the load created by the water and sediments filling the basin creates additional load, thus causing additional lithospheric flexure and amplifying the original subsidence that created the basin, regardless of the original cause of basin inception.<ref name="AllenandAllen" />

=== Thermal subsidence ===
Cooling of a lithospheric plate, particularly young oceanic crust or recently stretched continental crust, causes [[thermal subsidence]]. As the plate cools it shrinks and becomes denser through [[Negative thermal expansion|thermal contraction]]. Analogous to a solid floating in a liquid, as the lithospheric plate gets denser it sinks because it displaces more of the underlying mantle through an equilibrium process known as [[isostasy]].

Thermal subsidence is particularly measurable and observable with oceanic crust, as there is a well-established correlation between the [[Seafloor depth versus age|age of the underlying crust and depth of the ocean]]. As newly-formed oceanic crust cools over a period of tens of millions of years. This is an important contribution to subsidence in rift basins, backarc basins and passive margins where they are underlain by newly-formed oceanic crust.

=== Strike-slip deformation ===
[[File:Pull apart basin.jpg|thumb|upright=1.35|Shematic diagram of a strike-slip tectonic setting with fault bends creating areas of transtension and transpression]]
In [[Strike-slip tectonics|strike-slip tectonic]] settings, deformation of the lithosphere occurs primarily in the plane of Earth as a result of near horizontal maximum and minimum [[Stress (mechanics)#Principal stresses and stress invariants|principal stresses]]. Faults associated with these plate boundaries are primarily vertical. Wherever these vertical fault planes encounter bends, movement along the fault can create local areas of compression or tension.

When the curve in the fault plane moves apart, a region of ''[[transtension]]'' occurs and sometimes is large enough and long-lived enough to create a sedimentary basin often called a [[pull-apart basin]] or strike-slip basin.<ref name=BusbyIngersoll/> These basins are often roughly rhombohedral in shape and may be called a ''rhombochasm''. A classic rhombochasm is illustrated by the [[Dead Sea]] rift, where northward movement of the [[Arabian Plate]] relative to the [[Anatolian Plate]] has created a strike slip basin.

The opposite effect is that of ''[[transpression]]'', where converging movement of a curved fault plane causes collision of the opposing sides of the fault. An example is the [[San Bernardino Mountains]] north of Los Angeles, which result from convergence along a curve in the [[San Andreas Fault]] system. The [[Northridge earthquake]] was caused by vertical movement along local thrust and reverse faults "bunching up" against the bend in the otherwise strike-slip fault environment.