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Models of Extensional Basins
Styles of Continental Rifting: Results from Dynamic Models of Lithospheric Extension
Results are presented from models of extension, necking and rifting of the continental lithosphere. Two fundamentally different plate tectonic driving forces are considered: the gravitational force from uplift created by sub-lithospheric convection (local gravity push), and the tensile stress transmitted through the lithosphere as stress guide from subducted oceanic lithosphere (distant pull).
The thermo-mechanical finite element model represents the lithosphere as an elasto-visco-plastic continuum in which the rheological behaviour is determined by composition, temperature, pressure, the state of deviatoric stress and its rate of change. The composition of the crust and mantle are assumed to be dominated by wet quartz and wet olivine, respectively. Nonlinear effects from the complex rheology, large displacements and large deformation are included, as are the conductive and advective heat transfer, buoyancy forces and the first-order effects of erosion and sedimentation.
The local gravity push model predicts early doming of the rift zone followed by a slow subsidence phase. The width of the region in which extension takes place mirrors the width of the uplift stress.
Lithospheric bending effects are only important when the wavelength of the uplift stress applied at the base of the lithosphere is not significantly greater than the flexural thickness of the continental lithosphere.
The distant pull model predicts concomitant subsidence of the rift axis and uplift of the shoulders. The rift morphology may reflect the properties of pre-existing zones of lithospheric weakness. The amount of subsidence/uplift predicted by the model depends on the magnitude of the distant pull and the strength contrast between the assumed pre-weakened regions and the adjacent unaltered lithosphere.
Some contemporary rift zones show characteristics of each of the two rifting styles predicted by the model.
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