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Sandwell, D, Schubert G.  2010.  A contraction model for the flattening and equatorial ridge of Iapetus. Icarus. 210:817-822.   10.1016/j.icarus.2010.06.025   AbstractWebsite

Others have explained the excess flattening of Iapetus by a model in which the moon formed at a high spin rate, achieved isostatic equilibrium by very rapid interior heating caused by short-lived radioactive isotopes (SLRI), and subsequently cooled, locking in the excess flattening with respect to an equilibrium shape at its present spin rate. Here we propose an alternate model that does not require an unusually high initial spin rate or the SLRI. The initial formation of Iapetus results in a slightly oblate spheroid with porosity >10%. Radioactive heating by long-lived isotopes warms the interior to about 200 K, at which point it becomes ductile and the interior compacts by 10%, while the 120 km-thick exterior shell remains strong. The shell must deform to match the reduced volume of the ductile interior, and we propose that this deformation occurs along the equator, perhaps focused by a thinner equatorial shell. The final shape of the collapsed sphere matches the observed shape of Iapetus today, described as an oblate ellipse, except along the equator where strain concentration forms a broad ridge. To maintain this non-equilibrium shape, the thickness of the shell must exceed 120 km. Testing the equatorial focusing hypothesis will require a model that includes non-linear processes to account for the finite yield strength of the thick lithosphere. Nevertheless, we show that the stress in the lithosphere generated by the contraction of the interior is about 3 times greater than the stress needed to deform the lithosphere, so some type of lithospheric deformation is expected. (C) 2010 Elsevier Inc. All rights reserved.

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.

Smith, WHF, Sandwell DT.  1997.  Global sea floor topography from satellite altimetry and ship depth soundings. Science. 277:1956-1962.   10.1126/science.277.5334.1956   AbstractWebsite

A digital bathymetric map of the oceans with a horizontal resolution of 1 to 12 kilometers was derived by combining available depth soundings with high-resolution marine gravity information from the Geosat and ERS-1 spacecraft. Previous global bathymetric maps lacked features such as the 1600-kilometer-long Foundation Seamounts chain in the South Pacific. This map shows relations among the distributions of depth, sea floor area, and sea floor age that do not fit the predictions of deterministic models of subsidence due to lithosphere cooling but may be explained by a stochastic model in which randomly distributed reheating events warm the lithosphere and raise the ocean floor.

Levitt, DA, Sandwell DT.  1996.  Modal depth anomalies from multibeam bathymetry: Is there a south Pacific superswell? Earth and Planetary Science Letters. 139:1-16.   10.1016/0012-821x(95)00247-a   AbstractWebsite

A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called 'South Pacific Superswell' are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close. Depth-age results indicate that there is only a small anomaly (< 200 m) over 15-35 Ma Pacific Plate seafloor with little age-dependent shallowing, suggesting that the lithosphere east of the main hot-spot locations on the 'superswell' is normal. An important implication is that, in sparsely surveyed areas, depths from ETOPO-5 are significantly different from true depths even at large scales (similar to 1000 km) and thus are unsuitable for investigations of anomalies associated with depth-age regressions. We find that seafloor slopes on conjugate profiles of the Pacific and Nazca Plates from 15 to 35 Ma are both slightly lower than normal, but are within the global range. Proximate to the SEPR, seafloor slopes are very low (218 m Myr(-1/2)) on the Pacific Plate (0-22 Ma) and slightly high (similar to 410 m Myr(-1/2)) on the Nazca Plate (0-8 Ma); slopes for older Pacific seafloor (22-37 Ma) are near normal (399 m Myr(-1/2)). Seafloor slopes are even lower north of the Marquesas Fracture Zone but are highly influenced by the Marquesas Swell. We find that the low subsidence rate on young Pacific seafloor cannot be explained by a local hot-spot or a small-scale convective model exclusively and a stretching/thickening model requires implausible crustal thickness variation (similar to 30%).

Schubert, G, Sandwell DT.  1995.  A Global Survey of Possible Subduction Sites on Venus. Icarus. 117:173-196.   10.1006/icar.1995.1150   AbstractWebsite

About 10,000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on Venus. All the sites have narrow deep trenches elongate along strike with arcuate planforms, ridge-trench-outer rise topographic profiles typical of terrestrial subduction zones, large outer rise curvatures >10(-7) m(-1), fractures parallel to the strike of the trench on the outer trench wall and outer rise, and no cross-strike fractures across the trench. Both the northern and southern margins of Latona Corona are possible subduction sites. Identification of a major graben between the two principal outer ridges in southern Latona Corona is evidence of back-are extension in the corona; the amount of extension is estimated to be more than 2-11 km. The moment exerted by the ridges of southern Latona Corona is insufficient to bend the lithosphere into the observed outer rise shape; a negatively buoyant subducted or underthrust slab is needed. Depending on the unknown trench migration rate, lithospheric subduction can make a significant contribution to mantle cooling on Venus. Venusian chasmata could have a dual character. They may be propagating rifts near major volcanic rises, and subduction trenches far from the rises in the lowlands. Subduction and rifting may occur in close proximity on Venus, unlike on Earth. Rifting induced by hotspots on Venus may be necessary to break the lithosphere and allow subduction to occur. Such a process could result in gradual lithospheric subduction or global, episodic overturn of the lithosphere. (C) 1995 Academic Press, Inc.

Sandwell, DT, Winterer EL, Mammerickx J, Duncan RA, Lynch MA, Levitt DA, Johnson CL.  1995.  Evidence for Diffuse Extension of the Pacific Plate from Pukapuka Ridges and Cross-Grain Gravity Lineations. Journal of Geophysical Research-Solid Earth. 100:15087-15099.   10.1029/95jb00156   AbstractWebsite

Satellite altimeter measurements of marine gravity reveal 100 to 200-km wavelength lineations over a wide area of the Pacific plate oriented roughly in the direction of absolute plate motion. At least three mechanisms have been proposed for their origin: small-scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere; diffuse N-S extension of the lithosphere resulting in lineated zones of extension (boudins); and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism. Recently, several chains of linear volcanic ridges have been found to be associated with the gravity lineations. Following ridgelike gravity signatures apparent in high-resolution Geosat gravity measurements, we surveyed a series of volcanic ridges that extend northwest from the East Pacific Rise flank for 2600 km onto 40 Ma seafloor. Our survey data, as well as radiometric dates on samples we collected from the ridges, provide tight constraints on their origin: (1) Individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion. (2) The ridges formed along a band 50 to 70-km-wide in the trough of one of the more prominent gravity lineations. (3) Radiometric dates of the largest ridges show no hotspot age progression. Moreover, the directions predicted for minihotspot traces older than 24 Ma do not match observed directions of either the gravity lineations or the ridges. Based on this last observation, we reject the minihotspot model. The occurrence of the ridges in the trough of the gravity lineation is incompatible with the small-scale convection model which would predict increased volcanism above the convective upwelling. We favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere. However, the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model. Perhaps the fault scarps are buried under more than 30 m of sediments and/or covered by the elongated ridges. Finally, we note that if ridge-push force is much smaller than trench-pull force, then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion.

Johnson, CL, Sandwell DT.  1994.  Lithospheric Flexure on Venus. Geophysical Journal International. 119:627-647.   10.1111/j.1365-246X.1994.tb00146.x   AbstractWebsite

Topographic flexural signatures on Venus are generally associated with the outer edges of coronae, with some chasmata and with rift zones. Using Magellan altimetry profiles and grids of venusian topography, we identified 17 potential flexure sites. Both 2-D cartesian, and 2-D axisymmetric, thin-elastic plate models were used to establish the flexural parameter and applied load/bending moment. These parameters can be used to infer the thickness, strength and possibly the dynamics of the venusian lithosphere. Numerical simulations show that the 2-D model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter. However, an axisymmetric model must be used to obtain a reliable estimate of load/bending moment. 12 of the 17 areas were modelled with a 2-D thin elastic plate model, yielding best-fit effective elastic thicknesses in the range 12 to 34 km. We find no convincing evidence for flexure around smaller coronae, though five possible candidates have been identified. These five features show circumferential topographic signatures which, if interpreted as flexure, yield mean elastic thicknesses ranging from 6 to 22 km. We adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on Venus are similar to, or lower than, strain rates on Earth. Many of the flexural signatures correspond to relatively high plate-bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow, respectively. For areas where the curvatures are not too extreme, the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere. The large amplitude flexures in Aphrodite Terra predict complete failure of the plate, rendering mechanical thickness estimates from these features unreliable. One smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data. Reliable mechanical thicknesses found by forward modelling in this study are 21 km-37 km, significantly greater than the 13 km-20 km predictions based on heat-flow scaling arguments and chondritic thermal models. If the modelled topography is the result of lithospheric flexure, then our results for mechanical thickness, combined with the lack of evidence for flexure around smaller features, are consistent with a venusian lithosphere somewhat thicker than predicted. Dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere, also consistent with low temperature gradients. Recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed. Corresponding time-scales for gravitational relaxation are 10(8)-10(9) yr, making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features. If dry olivine is also found to be stronger than previously believed, the mechanical thickness estimates for Venus will be reduced, and will be more consistent with the predictions of global heat scaling models.

Sandwell, DT, Schubert G.  1992.  Evidence for Retrograde Lithospheric Subduction on Venus. Science. 257:766-770.   10.1126/science.257.5071.766   AbstractWebsite

Annular moats and outer rises around large Venus coronae such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On Earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronae while compensating back-arc extension is occurring in the expanding coronae interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of eastern Aphrodite Terra.