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2 Study of sedimentary basins |
Methods of Formation
It is common to categorise sedimentary basins according to the perceived method of formation, creating such groups as foreland basins (caused by lithospheric flexure), back-arc basins (caused by lithospheric stretching) and pull-apart basins (caused by strike-slip deformation of the lithosphere). Mechanisms for formation of sedimentary basins are numerous and often fundamentally linked to the concepts of plate tectonics.
Lithospheric stretching
If the lithosphere is caused to extend, 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 away from the locus of extension, whilst the upper, cooler and more brittle crust will tend to fault 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 example of a basin caused by lithospheric stretching is the North Sea - also an important location for significant hydrocarbon reserves.
Lithospheric 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 foreland basins, such as the Po basin in Italy and the Spanish basins of the Pyrenees.
The study of sedimentary basins as a specific entity in themselves is often referred to as basin modelling. 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.
See also: isostasy, plate tectonics.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. Study of sedimentary basins