Modelling Sea Floor Spreading Initiation and Depth Dependent Stretching at Rifted Continental Margins

Depth dependent stretching, in which upper crustal extension is much less than that of the lower crust and lithospheric mantle, has been observed at both non-volcanic and volcanic margins and is not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. The timing of depth dependent stretching on the Norwegian margin suggests that depth dependent stretching of continental rifted margin lithosphere occurs during early sea-floor spreading rather than during pre-breakup rifting. These observations suggest that the main thinning of rifted margin lithosphere occurs during early sea-floor spreading rather than during pre-breakup rifting. Single-phase fluid-flow models have been applied successfully to sea-floor spreading at ocean ridges. A single-phase fluid-flow model of sea-floor spreading initiation has been developed to determine rifted continental margin lithosphere thinning and thermal evolution resulting from early sea-floor spreading. The ocean-ridge initiation model uses an isoviscous corner-flow stream-function solution (Batchelor 1967) to predict the divergent lithospheric and asthenospheric fluid-flow field associated with early sea-floor spreading. The thinning of the rifted continental lithosphere is calculated by material advection in the newly initiated ocean ridge fluid-flow field. The model may also include the effects of pre-breakup pure-shear stretching of continental lithosphere. Rifted margin lithosphere thinning and thermal evolution is dependent on ocean-ridge spreading rate (Vx), the mantle upwelling velocity beneath the ridge axis (Vz), and the pre-breakup lithosphere beta stretching factor. The developed model predicts the thinning of the upper crust, lower crust and lithospheric mantle of the continental margin, and the history of rifted margin subsidence, water depths and top basement heat-flow. The ocean-ridge fluid-flow models predict advection of continental lithosphere material and depth dependent stretching which is highly sensitive to the ratio of Vz/Vx. Convection modelling of sea-floor spreading initiation at volcanic margins (Nielsen & Hopper 2002) suggests that Vz/Vx > 5 for early sea-floor spreading, reducing to Vz/Vx ~ 1.3 after a few Ma. Application of this Vz/Vx history predicts that lower continental crust and lithospheric mantle adjacent to the site of sea-floor spreading initiation is advected continentward producing a distribution of depth dependent stretching consistent with that observed at volcanic margins. Mantle exhumation is not predicted for Vz/Vx >>1. For non-volcanic margins, Vz/Vx at sea-floor spreading initiation is expected to be ~ 1. Models of sea-floor spreading initiation with Vz/Vx ~ 1 predict that lower continental crust and continental lithospheric mantle adjacent to the site of sea-floor spreading initiation are advected oceanward and result in rifted margin depth dependent stretching and continental mantle exhumation consistent with that observed at non-volcanic margins.