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2006
Watts, AB, Sandwell DT, Smith WHF, Wessel P.  2006.  Global gravity, bathymetry, and the distribution of submarine volcanism through space and time. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004083   AbstractWebsite

[ 1] The seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin. Relatively few of these features (< similar to 0.1%), however, have been dated, and so little is known about their tectonic setting. One parameter that is sensitive to whether a seamount formed on, near, or far from a mid-ocean ridge is the elastic thickness, T(e), which is a proxy for the long-term strength of the lithosphere. Most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of T(e) and then comparing the calculated and observed gravity anomaly. The problem with such an approach is that bathymetry data are usually limited to single-beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape. We therefore use the satellite-derived gravity anomaly to predict the bathymetry for different values of T(e). By comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the Wessel global seamount database, we have obtained 9758 T(e) estimates from a wide range of submarine volcanic features in the Pacific, Indian, and Atlantic oceans. Comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate T(e) and its upper and lower bounds. T(e) at sites where there is both a sample and crustal age show considerable scatter, however, and there is no simple relationship between T(e) and age. Nevertheless, we are able to tentatively assign a tectonic setting to each T(e) estimate. The most striking results are in the Pacific Ocean where a broad swath of "on-ridge'' volcanism extends from the Foundation seamounts and Ducie Island/Easter Island ridge in the southeast, across the equator, to the Shatsky and Hess rises in the northwest. Interspersed among the on-ridge volcanism are "flank ridge'' and "off-ridge'' features. The Indian and Atlantic oceans also show a mix of tectonic settings. Off-ridge volcanism dominates in the eastern North Atlantic and northeast Indian oceans, while flank ridge volcanism dominates the northeastern Indian and western south Atlantic oceans. We have been unable to assign the flank ridge and off-ridge estimates an age, but the on-ridge estimates generally reflect, we believe, the age of the underlying oceanic crust. We estimate the volume of on-ridge volcanism to be similar to 1.1 x 10(6) km(3) which implies a mean seamount addition rate of similar to 0.007 km(3) yr(-1). Rates appear to have varied through geological time, reaching their peak during the Late/Early Cretaceous and then declining to the present-day.

1991
Marks, KM, Sandwell DT.  1991.  Analysis of Geoid Height Versus Topography for Oceanic Plateaus and Swells Using Nonbiased Linear-Regression. Journal of Geophysical Research-Solid Earth and Planets. 96:8045-8055.   10.1029/91jb00240   AbstractWebsite

We have investigated the relationship between geoid height and topography for 53 oceanic plateaus and swells to determine the mode of compensation. The ratio of geoid height to topography was obtained from the slope of a best line fit by functional analysis (i.e. nonbiased linear regression), a method that minimizes both geoid height and topography residuals. This method is more appropriate than traditional least squares analysis that minimizes only geoid height residuals, because uncertainties are present in both data types. We find that approximately half of the oceanic and continental plateaus analyzed have low ratios that are consistent with Airy-compensated crustal thickening. The remaining plateaus, however, have higher geoid/topography ratios than predicted by the simple Airy model, and the seismically determined Moho depths beneath some of these features are too shallow for crustal thickening alone. A two-layer Airy compensation model, composed of thickened crust underlain by an anomalously low density "mantle root", is used to explain these observations. The Walvis Ridge, and the Agulhas, Crozet, and north Kerguelen plateaus have geoid/topography ratios and Moho depths that are consistent with the two-layer Airy model. The proximity of the Agulhas Plateau to a RRR triple junction during its early development, and the excessive volcanism at active spreading ridges that created the Crozet and north Kerguelen plateaus and the Walvis Ridge, may have produced regions of enhanced depletion and hence the low-density mantle anomalies. If this explanation is correct, then the low-density mantle anomaly persists over time and remains embedded in the lithosphere beneath the oceanic feature.

Marks, KM, Sandwell DT, Vogt PR, Hall SA.  1991.  Mantle Downwelling beneath the Australian-Antarctic Discordance Zone - Evidence from Geoid Height Versus Topography. Earth and Planetary Science Letters. 103:325-338.   10.1016/0012-821x(91)90170-m   AbstractWebsite

The Australian-Antarctic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120-degrees and 128-degrees-E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling and/or cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geoid/topography ratios of 2.1 +/- 0.9 m/km for the entire study area (38-degrees-60-degrees-S, 105-degrees-140-degrees-E), 2.3 +/- 1.8 m/km for a subset comprising crust less-than-or-equal-to 25 Ma, and 2.7 +/- 2.0 m/km for a smaller area centered on the AAD were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m/km), and 2.7 m/km is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, estimated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the AAD is below the average depth of the base of the young lithosphere (approximately 30 km), and a downwelling of asthenospheric material is implied. The observed geoid height-age slope over the discordance is unusually gradual at -0.133 m/m.y. We calculate that an upper mantle 170-degrees-C cooler and 0.02 g/cm3 denser than normal can explain the shallow slope. Unusually fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.

Sandwell, DT.  1991.  Geophysical Applications of Satellite Altimetry. Reviews of Geophysics. 29:132-137. AbstractWebsite
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