Viscous overthrusting versus folding: 2-D quantitative modeling and its application to the Helvetic and Jura fold and thrust belts

This paper presents two-dimensional (2-D) numerical simulations of the shortening of a stiff viscous layer, with a pre-existing weak zone, that is embedded in a weaker viscous matrix. The model bottom remains straight during shortening and represents a detachment surface. Four deformation styles are observed that depend on the ratio of the layer and matrix viscosities, μL/μM; the thickness ratio of the bottom matrix and the overlying stiff layer, HM/HL; and the power-law stress exponent of the matrix, nM. The numerical results are used to quantify the conditions for which each deformation style occurs: (1) pure shear-dominated deformation occurs for μL/μM < ∼50 and nM = 1 (i.e., linear viscous); (2) overthrusting-dominated deformation occurs for a power-law viscous matrix (nM = 5), μL/μM > ∼50 and HM/HL < ∼0.5; (3) folding-dominated deformation occurs for nM = 1, μL/μM > ∼50, and HM/HL > ∼1; and (4) folding and overthrusting occur for a power-law viscous matrix and ∼0.5 < HM/HL < ∼2. The power-law stress exponent of the stiff layer has a minor effect on the deformation style. Simulations with layers that contain two weak zones show the formation of a nappe stack. The simulations also show that multi-layers can overthrust like a single layer. The change in deformation style as a function of HM/HL corroborates field observations from the Helvetic nappe system. The agreement of the numerical results with first-order observations from the Helvetic nappe system suggest that ductile deformation dominated this fold and thrust belt. The results further suggest that overthrusting on an effectively viscous weak layer only occurs if the rheology of the weak layer is power-law viscous.