Modelling Sag Basin Development and Rifted Margin Asymmetry During Continental Breakup and Rifted Margin Formation

Nick J. Kusznir and Roseanna J. Fletcher
Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom

Both non-volcanic and volcanic rifted continental margins, including conjugate margin pairs, show depth-dependent lithosphere thinning and stretching. Depth-uniform (pure-shear) models of continental lithosphere thinning leading to breakup fail to explain observed depth-dependent lithosphere stretching and the exhumation of continental lithosphere mantle. Decoupled pure-shear models of lithosphere thinning leading to continental breakup, while able to predict depth-dependent lithosphere stretching, require significantly larger amounts of upper crustal extension and faulting than is observed. A new model has been developed in which continental lithosphere thinning is achieved by a simultaneous combination of pure-shear and induced upwelling divergent flow within continental lithosphere and asthenosphere driven by thermal and melt buoyancy initiated by pure-shear lithosphere stretching. While horizontal tensile plate forces provide the driving force for the pure-shear deformation, the induced upwelling divergent flow provides the main contribution to continental lithosphere thinning. The new model successfully predicts depth-dependent stretching of continental margin lithosphere for both non-volcanic and volcanic margins, mantle exhumation at non-volcanic margins, an explanation for the observed paucity of pre-breakup brittle deformation, and a simple transition from pre-breakup lithosphere thinning to sea-floor spreading. The induced upwelling divergent flow model predicts the formation of pre-breakup sag basins and asymmetric conjugate rifted margins. Observed bathymetry and gravity anomalies predicted by the new margin formation model have been used to invert for kinematic parameters describing breakup lithosphere deformation and to predict rifted margin lithosphere structure, OCT location, subsidence and heat-flow history.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery