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21st Bullerwell Lecture When: Tuesday evening, 27 March 2001 Where: XXVI General Assembly of the European Geophysical Society, Acropolis, Nice (See http://www.copernicus.org/EGS/egsga/nice01/nice01.htm) Interrogating Sedimentary Basins Dr Nicky White, Department of Earth Sciences, Cambridge University 70% of the Earth's surface is covered in >2 km of sedimentary rock and the largest accumulations occur on rifted continental margins. Sedimentary basins are of obvious economic significance but they also contain a wealth of information about the temporal and spatial changes caused by a variety of geological processes. Superb three-dimensional seismic images of the structure and fill of basins such as the North Sea are now available. As geophysicists, we are interested in extracting quantitative information about Earth processes, preferably by inverse modelling, from these spectacular data. How do basins form? Although it is generally accepted that many basins are generated by thinning of the lithospheric plate, the way in which basins grow through time and space has received little attention. It is now opportune to address this particular problem by analysing three-dimensional seismic datasets. Here, I describe and apply an inverse algorithm which extracts the spatial and temporal pattern of strain rate from sedimentary basins. At present, only the two-dimensional problem is tractable but my approach can be regarded as a stepping stone towards a generalised three-dimensional inversion. My starting point is a simple forward model which allows basin stratigraphy to be calculated from any given strain rate distribution. This forward model includes potentially important effects such as the lithosphere's elastic thickness and the two-dimensional conduction/advection of heat. Conversely, inverse modelling determines strain rate variation by minimising the misfit between predicted and observed patterns of basin subsidence. No prior assumptions about the number, duration or intensity of rifting episodes are necessary. Instead, strain rate is allowed to vary smoothly throughout time and space. I have successfully inverted different synthetic sedimentary basin geometries which were generated by forward modelling. Sensitivity analysis demonstrates that strain rate patterns can be recovered with confidence. The relationship between stratigraphic misfit and elastic thickness shows that usually an upper limit, but not a lower limit, for elastic thickness can be retrieved. This inversion algorithm has been applied to Phanerozoic sedimentary basins located worldwide. In each example, observed subsidence profiles can be automatically and accurately fitted. The calculated distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes. Peak strain rate estimates are comparable to present-day measurements in actively extending basins such as the Aegean Sea and the Basin and Range Province. These estimates will help to constrain the dynamical evolution of thinning continental lithosphere. Strain rate patterns also govern the heatflow history and can be used to construct accurate three-dimensional maturation models. Our results indicate that acceptable misfits are achieved when the elastic thickness of the lithosphere is less than 2-4 km. This surprising observation suggests that the lithosphere beneath sedimentary basins remains weak both during and after rifting. It also underscores the importance of inverting datasets so that the existence, uniqueness, and resolution of solutions can be analysed. In summary, three-dimensional seismic imaging of sedimentary basins has generated a rich variety of sub-surface data whose full potential has yet to be exploited by geophysicists. There are many important and tractable problems which can be addressed using these data. Here, I propose one line of enquiry which uses an inverse approach to extract information about lithospheric thinning.