We use thermo-mechanical finite element model experiments to investigate factors that are potentially important controls during volcanic and non-volcanic passive margin formation which may explain these characteristic differences. Our focus is on processes that create shear zones, on the rheological stratification of the lithosphere, and on processes that lead to differential thinning of upper and lower lithosphere during rifting. Dynamic modelling cases are compared where the crust is strong, weak, or very weak, and the mantle lithosphere is either strong or weak. Strain softening takes the form of a reduction in the internal angle of friction with increasing strain. Predicted rift modes belong to three fundamental types: 1) narrow, asymmetric rifting in which the geometry of both the upper and lower lithosphere is approximately asymmetric; 2) narrow, asymmetric, upper lithosphere rifting concomitant with narrow, symmetric, lower lithosphere extension; and 3) wide, symmetric, crustal rifting concomitant with narrow, mantle lithosphere extension.