| Convective circulation of the Earth’s mantle maintains plate motion but we know little about
the spatial and temporal details of this circulation. Fortunately, the circulation pattern beneath
lithospheric plates generates dynamic topography at the Earth’s surface. 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.
However, dynamic topography is difficult to measure because crustal and lithospheric
thickness and density changes are the dominant control of surface elevation. As a
result, smaller dynamic signals are swamped. Important progress can be made by
exploiting our quantitative understanding of the thermal evolution of oceanic lithosphere
(Winterbourne et al., 2009). We have analyzed seismic reflection and wide-angle profiles
from continental margins worldwide. First, we calculate the water-loaded depth of
old oceanic lithosphere adjacent to each margin. Secondly, we estimate residual
depth anomalies by comparing our water-loaded depths to the global age-depth
curve. These residual depth anomalies are then compared with dynamic topographic
predictions made from long-wavelength gravity anomalies. Positive and negative
deviations from the global age-depth curve are common and have amplitudes of
±1 km and wavelengths of 102–104 km. They mostly, but not always, correlate
with long-wavelength gravity anomalies. 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, by exploiting the mid-oceanic ridge
system, and by examining the age-depth relationship in marginal oceanic basins.
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.
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 key information about surface
elevation changes if sea-level variation is known. In several cases, well-calibrated
seismic surveys can be used to constrain spatial and temporal patterns of dynamic
topography. For example, there is excellent evidence for convectively driven domes
along the western margin of Africa. These domes grew rapidly during Neogene
times. Finally, Roberts & White (2010) have shown that longitudinal river profiles
can be inverted as a function of regional uplift rate history. If used with care, this
method can be used to map spatial and temporal patterns of dynamic topography
continent-wide.
Roberts, G. & White, N., 2010. Estimating uplift rate histories from river profiles using
African examples. Journal of Geophysical Research, 115, B02406.
Winterbourne, J., Crosby, A., & White, N., 2009. Depth, age and dynamic topography of
oceanic lithosphere beneath heavily sedimented Atlantic margins. Earth and Planetary
Science Letters, 287, 137–151. |