Sedimentary basin
The term
sedimentary basin is used to refer to any geographical feature exhibiting
subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing
pressure and begin the process of
lithification.
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).
Lithospheric stretching
If the
lithosphere is 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
hydrocarbon reserves. Another such feature is the
Basin and Range province which covers most of the USA state of Nevada, forming a series of
horst and
graben structures.
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 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 Islands chain of volcanic edifices has sufficient mass to cause deflection in the lithosphere.
The
obduction of 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
Pyrenees in Spain.
Strike-slip deformation
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 Sea rift, where northward movement of the
Arabian Plate relative to the
Anatolian Plate has 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 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.
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.
The study of sedimentary basins as a specific entity in themselves is often referred to as
basin modelling or
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
hydrocarbon reserves and as such are the focus of intense commercial interest.
*
isostasy*
plate tectonics
