| Convective circulation of the Earth’s mantle maintains plate motion but we know little about
the spatial and temporal details of this circulation. Accurate maps of the spatial and temporal
pattern of dynamic topography will profoundly affect our understanding the the relationship
between surface geology and deep Earth processes. A major difficulty is the ‘tyranny of
isostasy’. In other words, dynamic topography is difficult to measure because crustal and
lithospheric thickness and density changes are the dominant control of surface elevation.
Some progress can be made along continental margins by measuring residual depth
anomalies of the oldest oceanic floor on newly available seismic reflection and wide-angle
profiles. These estimates of dynamic topography have amplitudes of ±1 km and wavelengths
of 102–104 km. They mostly, but not always, correlate with long wavelength free-air
gravity anomalies. Correlation with seismic tomographic images is much poorer. The
distribution of dynamic topography throughout the rest of the oceanic realm can be
supplemented by using ship-track data in regions with sparse sedimentary cover and by
exploiting the mid-oceanic ridge system. On the continents, it is more difficult to
measure dynamic topography with the same accuracy since the density structure of
continental lithosphere is so variable but progress can be made on three fronts.
First, long-wavelength gravity anomalies which straddle continental margins are
an obvious and important guide. Secondly, stratal geometries across continental
shelves contain information about positive and negative surface elevation changes. In
several cases, 2- and 3-D seismic surveys calibrated by boreholes can be used to
constrain spatial and temporal patterns of dynamic topography. In the North Atlantic
Ocean, examples of buried ephemeral landscapes suggest that dynamic topography
can grow and decay on timescales as short as a few million years. Recognition of
positive and negative vertical motions, which cannot be accounted for by global
eustasy, is encouraging and suggests that we are on the verge of creating global
dynamic topographic maps which can be used to test predictive global models. |