3 - Models for the evolution of passive margins

Abstract

Backstripping studies show that the subsidence and uplift of rifted passive margins is mainly controlled by sediment and water loading, and thermal contraction and uplift. Heat flow data suggest that young margins generally have higher heat flow than surrounding cratonic areas whereas at old margins it is either similar, or less. This is in accord with elastic thickness data which show that young margins have lower values of the elastic thickness than old margins, suggesting that as the sub-crustal mantle cools following heating at the time of rifting, it increases in strength. Many passive margins, however, have low elastic thickness, irrespective of the age since rifting suggesting that other processes such as magmatism, load-induced increases in curvature and yielding, and sediment blanketing may act to weaken sub-crustal mantle. Numerical and analogue models have provided new constraints on the tectonic factors that control the width, structural style, and along-strike segmentation of passive margins. While pre-existing zones of weakness have been shown to localise strain in rifts, tectonic stresses appear to be large enough to cause stable lithosphere to neck almost to the point of rupture. Necking is modulated by processes such as magmatism that localise strain and processes, such as viscous flow, crustal buoyancy and flexure that de-localise it. The net result is that while most conjugate passive margin pairs show a high degree of symmetry in their large-scale crust and mantle structure, they also show asymmetry, especially in their faulting styles, syn-rift and post-rift stratigraphy, and thermal and mechanical properties. Current studies are re-examining the extent to which plate processes such as flexure and thermal contraction and uplift contribute to the subsidence and uplift of passive margins, compared to that of other processes such as those associated with motions in the sub-crustal mantle.