Effect of plastic-viscous layering and strain softening on mode selection during lithospheric extension

[1] Factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods. We view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer. The rate of energy dissipation is analyzed for pure shear (PS), symmetric plug (SP), and asymmetric plug (AP) extension modes, and the analysis reveals that the primary control is the relative rate of dissipation in the two layers. A basic difference is that the plastic layer yield strength is independent of the strain rate, whereas the viscous stress depends on strain rate; therefore dissipation scales linearly and quadratically with extension velocity for these respective layers. When other parameters, e. g., extension velocity, and properties are held constant, minimum dissipation predicts that the modes AP, SP, and PS will be selected in this order with increasing viscosity of the lower layer. Transition viscosities between modes, eta(T1) and eta(T2) are 4 x 10(21) Pa s and 8 x 10(22) Pa s, respectively, for our parameters values. Numerical models confirm the analysis results, inferred mode controls, and order of mode selection when strain softening of the plastic layer occurs during extension. Implications for lithosphere that acts as a bonded plastic/viscous laminate include the following (1) asymmetric extension (AP mode) is preferred when the extension rate and/or effective viscosity is low or the viscous region temperature is high, (2) symmetric (SP mode) extension is preferred for intermediate combinations of parameters, and (3) overall pure shear (PS mode) may occur for opposite end-member parameter combinations.