- Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting
subsidenceand consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressureand begin the process of lithification.
Methods of Formation
It is common to categorise sedimentary basins according to the mechanism of formation: tectonic compression (e.g., "foreland" basins, caused by lithospheric flexure), tectonic extension (e.g., "back-arc" basins, caused by lithospheric stretching), and tectonic strike-slip (such as "pull-apart" basins).
lithosphereis caused to stretch horizontally, by mechanisms such as "ridge-push" or "trench-pull", 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 will tend to fault (crack) and fracture. The combined effect of these two mechanisms is for the 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 might be 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 hydrocarbonreserves. Another such feature is the Basin and Rangeprovince which covers most of the USA state of Nevada, forming a series of horst and grabenstructures.
Another expression of lithospheric stretching results in the formation of ocean basins with central ridges; The
Red Seais in fact an incipient ocean, in a plate tectonic context. The mouth of the Red Sea is also a tectonic triple junctionwhere the Indian Ocean Ridge, Red Sea Rift and East African Great Rift Valley meet. This triple junction is also the only place on the planet where seafloor crust is subaerially exposed. The reason for this is twofold, due to a high thermal buoyancy of the junction, and a local crumpled zone of seafloor crust acting as a dam against the Red Sea.
Lithospheric compression/shortening and flexure
If a load is placed on the lithosphere, it will tend to flex in the manner of an elastic plate. The rate and degree of flexure is a function of the "
flexural rigidity" of the lithosphere, which is itself a function of the lithospheric mineral composition and thermal regime. The nature of the load is varied. For instance, the Hawaiian Islandschain of volcanic edifices has sufficient mass to cause deflection in the lithosphere.
obductionof one tectonic plate onto another also causes a load and often results in the creation of a foreland basin, such as the Po basin next to the Alps in Italy, the Molasse Basin next to the Alps in Germany, or the Ebro basin next to the Pyreneesin Spain.
Deformation of the lithosphere in the plane of the earth (i.e. such that faults are vertical) occurs as a result of horizontal differential stresses. The resulting zones of subsidence are known as strike-slip or pull-apart basins. Basins formed through strike-slip action occur where a vertical fault plane curves. When the curve in the fault plane moves apart, a region of "transtension" results, creating a basin. Another term for a transtensional basin is a "rhombochasm". A classic rhombochasm is illustrated by the
Dead Searift, where northward movement of the Arabian Platerelative to the Anatolian Platehas caused a rhombochasm.
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 Mountainsnorth of Los Angeles, which result from convergence along a curve in the San Andreas faultsystem. The Northridge earthquakewas caused by vertical movement along local thrust and reverse faults "bunching up" against the bend in the otherwise strike-slip fault environment.
Ongoing development of sedimentary basins
As more and more sediment is deposited into the basin, the weight of all the newer sediment may cause the basin to subside further because of
isostasy. A basin can continue having sediment deposited into it, and continue to subside, for long periods of geological time; this can result in basins many kilometres in thickness. Geologic faults can often occur around the edge of, and within, the basin, as a result of the ongoing slippage and subsidence.
Study of sedimentary basins
The study of sedimentary basins as a specific entity in themselves is often referred to as
basin modellingor Sedimentary Basin Analysis. The need to understand the processes of basin formation and evolution are not restricted to the purely academic. Indeed, sedimentary basins are the location for almost all of the world's hydrocarbonreserves and as such are the focus of intense commercial interest.
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