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Plane-strain, thermo-mechanical, finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional-plastic regime and the strength of the lower crust and mantle lithosphere, respectively, on the style of extension. Crust and mantle lithosphere strength are varied independently. A simple scaling of wet quartz and dry-olivine rheologies is used to examine crust and mantle lithosphere strength variations. Cases are compared where the crust is strong (ηwet quartz x 100), weak (ηwet quartz), or very weak (ηwet quartz/10), and the mantle lithosphere is either strong (ηdry olivine) or weak (ηdry olivine/10). 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. The different styles depend on the relative control of the system by the frictional-plastic and ductile layers, which promote narrow, localized rifting in the plastic layers and wide modes of extension in the viscous layers, respectively. A weak, ductile crust-mantle coupling tends to suppress narrow rifting in the crustal layer. This is because it reduces the coupling between the frictional-plastic upper crust and localized rifting in the frictional-plastic upper mantle lithosphere. The simple strength variation may be taken to represent end-member thermal and/or compositional conditions in natural systems and the relevance for rifting of old, strong, and cold cratonic lithosphere as compared to young, “standard”, and moderately weak Phanerozoic lithosphere is discussed.

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