Roles of lithospheric strain softening and heterogeneity in determining the geometry of rifts and continental margins
R. S. Huismans, C. Beaumont, 2007. "Roles of lithospheric strain softening and heterogeneity in determining the geometry of rifts and continental margins", Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup, G. D. Karner, G. Manatschal, L. M. Pinheiro
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Plane strain thermo-mechanical finite-element model experiments are used to investigate the effects of frictional–plastic strain softening and inherited weakness on the style of lithospheric extension. The model results are compared with the Newfoundland–Iberia conjugate rifted margins with the goal of understanding the lithospheric properties that controlled their evolution during rifting. Our proposition is that coupling between the plastic–viscous layering, acting together with frictional–plastic strain softening localized on inherited weak heterogeneities, can explain the initial wide rift and distributed rift basins that are later abandoned in favour of a narrow rift in which mantle lithosphere is exhumed to the surface. The models comprise uniform composition viscous and plastic layers in which focused deformation is nucleated on either a single weak ‘seed’ or a statistical white noise distribution of inherited strain. Strain softening of frictional–plastic layers acts as a positive feedback mechanism that creates localized shear zones from the inherited weak heterogeneities. The sensitivity of deformation to the choice of softening parameters and the type of inherited noise is examined in cases where the deeper part of the crust is either weak or strong.
Lithosphere-scale models with a single weak seed exhibit a range of asymmetric and symmetric rifting modes that are mostly determined by the feedback between two primary controls, coupling between the plastic and viscous layers and strain softening. Decreasing and increasing the rifting velocity can change the mode, and asymmetry is strongest in models with low rifting velocities and a strong lower crust. Analysis of equivalent simple-bonded plastic–viscous two-layer models using the minimum rate of dissipation principle demonstrates that the mode selected depends on the division of the dissipation between the layers. Criteria developed on minimizing the total dissipation show how mode selection changes with increasing viscosity, or rifting velocity, from the: asymmetric plug or half-graben (AP) mode; through the symmetric plug or graben (PS) mode, to the distributed pure shear (PS) mode. Numerical models confirm these results.
Models with statistical white-noise-inherited strain have similar modes to those with a single seed. In addition, modes with multiple sets of shear zones develop in the plastic layer for a range of intermediate parameter combinations. We believe that distributed noise in combination with a weak lower crust and slow extension can produce model results in accord with general features of the Newfoundland–Iberia conjugate margins; an initially distributed wide rift mode, followed by a late-stage narrow rift with a significant component of mantle exhumation.
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This book summarizes our present understanding of the formation of passive continental margins and their ocean–continent transitions. It outlines the geological, geophysical and petrological observations that characterize extensional systems, and how such observations can guide and constrain dynamic and kinematic models of continental lithosphere extension, breakup and the inception of organized sea-floor spreading. The book focuses on imaging, mapping and modelling lithospheric extensional systems, at both the regional scale using dynamic models to the local scale of individual basins using kinematic models, with an emphasis on capturing the extensional history of the Iberia and Newfoundland margins. The results from a number of other extensional regimes are presented to provide comparisons with the North Atlantic studies; these range from the Tethyan realm and the northern Red Sea to the western and southern Australian margins, the Basin and Range Province, and the Woodlark basin of Papua New Guinea. All of these field studies, combined with lessons learnt from the modelling, are used to address fundamental questions about the extreme deformation of continental lithosphere.