Revisiting fundamental concepts of continental extension
Depth-uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup; it cannot explain the observed depth-dependent lithosphere stretching and mantle exhumation at rifted continental margins. Depth-dependent lithosphere thinning, in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust, has been observed at many non-volcanic and volcanic rifted continental margins including conjugate margin pairs. Passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea-floor spreading initiation. A new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere, and that this deformation evolves into sea-floor spreading. The new model successfully predicts depth-dependent stretching of continental margin lithosphere for both non-volcanic and volcanic margins and mantle exhumation at non-volcanic margins, both of which are observed, but are not explained, by existing depth-uniform continental lithosphere stretching models. The new model provides a balance of extensional strain, supplies an explanation for the paucity of synrift brittle deformation, and offers a simple transition from prebreakup lithosphere thinning to sea-floor spreading. The observed diversity of rifted continental margin structure and width of the ocean–continent transition can be explained by variability in the form of the upwelling divergent flow field. The new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea-floor spreading initiation in the Woodlark Basin in the western Pacific, and the observed bathymetry and free air gravity anomaly for the Newfoundland and Iberian margins.
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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.