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Crustal deformation during the extension of a continental
half-graben
Overview. This is a 2D mechanical experiment showing
how crustal deformation occurs in the crust during its extension. The initial
geometry of a high angle pre-existing fault is assumed.
Model parameters are defined in files iessai
and jessai . An elastoplastic model (Drucker-Prager plasticity) and a
non linear viscoelastic model
(Maxwell model) are used to model respectively the frictional crust and the
ductile crust. Fault motion is
accounted by frictional contact (Signorini and Coulomb laws). A friction
coefficient of 0.1 is used in this model . Model boundary conditions are 1)
extensive lateral velocities imposed on vertical sides (figure); 2) hydrostatic pressure at the base of the crust
(he mantle is assumed to have a low viscosity). A laterally homogeneous thermal
field is assumed, corresponding to a high temperature gradient of 20°C/km.
Results. After about 1.5 Myr, a high deviatoric stress
is generated in the upper crust, while a decay occurs in the lower crust due to
temperature dependent viscous properties. Highest stress are located at about 12
km depth. Stress perturbation (magnitude and axis) appear near the fault (figure).
According to extension and low fault friction, localized fault slip occurs and
generates footwall uplift and hanging wall subsidence (figure).
This motions corresponds to surface plastic deformation in the basin and deep
strain inside the lower plate (figure).
Due to the low viscous stress at depth, fault slip damps below 15 km and is
transferred inside the continuous medium.
Related publication. A numerical experiment
similar to this one has been used to explain the formation of metamorphic core
complexes (Chéry ,2001, pdf)
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