Effect of Lithospheric Stratification on Extensional Styles and Rift Basin Geometry
Published:December 01, 2005
Ritske S. Huismans, Christopher Beaumont, 2005. "Effect of Lithospheric Stratification on Extensional Styles and Rift Basin Geometry", Petroleum Systems of Divergent Continental Margin Basins, Paul J. Post, Norman C. Rosen, Donald L. Olson, Stephen L. Palmes, Kevin T. Lyons, Geoffrey B. Newton
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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.