Spatio-Temporal Evolution of Dynamic Topography
It has long been accepted that flow within the convecting mantle generates some proportion of the Earth's surface topography, inducing vertical motions that vary as a function of both space and time. This dynamic component of topography should be ubiquitous but observing it directly is complicated by the need to adequately account for isostatic contributions. This complication has led many workers to take a "bottom-up" approach, predicting the spatio-temporal evolution of dynamic topography through numerical models with assumed radial viscosity profiles and input density anomalies derived from seismic tomography. All outputs predict slowly varying dynamic topography with ± 2 km maximum amplitude and ~ 10, 000 km characteristic wavelengths.
However, recent work exploiting an extensive database reflection and refraction profiles has, for the first time, allowed dynamic topography to be constrained observationally. This dataset demonstrates that dynamic topography varies appreciably at wavelengths at least as short as ~ 1, 000 km and that, although the maximum amplitude is ± 2 km, the typical amplitude at ~ 10, 000 km wavelengths is just 1/4 that predicted by modelling studies.
Although we now have better constraint on present-day dynamic topography, determining its spatio-temporal evolution remains a major challenge. Nevertheless, determining the timescales on which dynamic topography evolves is crucial to constraining the viscosity structure of the Earth's mantle, a poorly known parameter at the centre of many outstanding geodynamic controversies. The principal aim of my PhD is to constrain this time-dependence by analysing the response of passive margin systems to mantle-derived perturbations.
Initial work has focussed on the western Indian continental margin. Oceanic residual depths show a clear long-wavelength (~ 2000 km) east-west asymmetry across the peninsula from highs of + 1 km in the west to lows of - 1.2 km in the east and this asymmetry is mirrored onshore with South Indian topography exhibiting a long-wavelength eastward tilt of similar gradient. By integrating evidence from low-temperature thermochronology, elevated marine deposits, dated palaeosurfaces, sediment flux histories and stratal architecture with inversions of Peninsular Indian drainage networks, a continental uplift history has been reconstructed which suggests that the present-day eastward tilt mainly developed after ~ 30 Ma. Vs anomalies in the shallow mantle are consistent with the present-day topographic asymmetry of the peninsula being principally controlled by temperature anomalies in a low-viscosity asthenospheric channel. This suggests that the Neogene uplift observed over India may relate to changes in the planform of upper mantle convection with respect to the Indian Plate since 30 Ma.
Professor Nicky White
Dr. Paul Bellingham (P. D. F. Limited)
Geochemistry ; Geophysics