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Thermal Subsidence

The initiation of Sedimentary Basins and Tectonic subsidence

     In this section the first few million years in the life of a sedimentary basin will be discussed.

* Firstly the forces believed to initiate lithospheric extension will be briefly look at.
* Then models of lithospheric extension by McKenzie and Wernicke will be looked at to give an overview of lithospheric stretching.
* Then the initial stage of subsidence in the formation of a sedimentary basin, tectonic subsidence will be examined.
* Finally calculating tectonic subsidence will be explored.

The origin of the extensional force

     This report is focusing on sedimentary basins formed from lithospheric stretching. Some of the processes that have been argued to cause the initial extensional force on the lithosphere include:

* Mantle convection, the material in the lower mantle is hotter and will therefore travel up to the surface debase it is less dense than colder material. It will then         cool in the upper mantle and traveling down again. The material will heat up in the lower mantle and rise. The process repeats to form a convection cell.

* Mantle plumes, a bouyant mass of hot mantle that rises to the base of the lithosphere. Mantle plumes commonly  produce volcanic activity and structural              deformation in the central parts of lithospheric plates (Hamblin 1998).

* Slab pull and ridge push,  in a subduction zone there is a force acting on the overiding plate from the creation and destruction of  lithosphere.

Models of lithospheric extension

     A wide variety of models have been proposed for how sedimentary basins originate and develop, however most have limited predictive power and rely on unobserved processes to operate in the crust and upper mantle. Two models for lithospheric extension have been widely excepted, one by McKenzie (1978) and one by Wernicke (1985). The McKenzie model (1978) is based on pure shear which results in symmetric basins whereas the Wernicke model (1985) is based on simple shear which results in asymmetric basins.

McKenzie Model (symmetric basins, uniform stretching, pure shear)

     In the pure shear model proposed by McKenzie there is an initial instantaneous (less than 20 My) stretching of the lower crust and upper crust. This is called the initial subsidence (Si) or tectonic subsidence.
The strain is distributed evenly across the continental lithosphere.
Crustal thinning will occur through ductile pure shear extension of the lower crust and lithospheric mantle and brittle normal faulting of the upper crust.
For isostatic equilibrium to be maintained the height of the aesthenoshere rises.
The initial subsidence is then followed by a much slower longer period of thermal subsidence. This is called the thermal subsidence, St.

Fig 2.1. The McKenzie model of lithospheric extension.

Wernicke Model (asymmetric basins, simple shear)

     In the Wernicke's simple shear model a low-angle detachment fault penetrates through the lithosphere. The detatchment fault allows for extension through relative displacement along the fault.
     The upper crusts deformation is brittle and the lower crusts deformation is ductile.
Basins associated with the thermal subsidence phase may be offset from the basin associated with the tectonic phase of subsidence.

Fig 2.2.  The Wernicke model of lithospheric extension.

Fig 2.6. Isostasy, cross sections in isostatic equilibrium.


Calculating tectonic subsidence

    In figure 7 the lithosphere has been divided into three layers (the upper crust, lower crust and the mantle), the subsidence or uplift at any point on the surface can be calculated by adding up the amount of displacement (net vertical movement) experienced in each of the layers. Subsidence associated with crustal thinning.

Fig 2.7. Lithospheric layers and formula parameter definitions.


Mantle contribution

     The extension in the lithospheric mantle is assumed to be accommodated by ductile pure shear. The mantles contribution to the net vertical movement at the surface can be calculated using the following formula created by McKenzie (1978) and McKenzie and White (1988).


Lower Crust contribution

     The extension in the lower crust is assumed to be totally ductile. The lower crusts contribution to the net vertical movement at the surface can be calculated using the following formula created by McKenzie (1978) and McKenzie and White (1988).


Upper Crust contribution

    Extension in the brittle upper crust is accommodated by pure shear along normal faults. The upper crusts contribution to the net vertical movement at the surface can be calculated using the following formula created by McKenzie (1978) and McKenzie and White (1988).


Example of tectonic subsidence calculation

     In the following example (figure 8) the tectonic subsidence has not been calculated for each layer of the lithosphere individually and then added together, the tectonic subsidence has been calculated in one step. This formula was also created by McKenzie.

Fig 2.8. Example of tectonic subsidence calculation.
     It is common to get a result of tectonic subsidence in the order of 1-2 kilometers.

Thermal Subsidence
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