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1980
Sandwell, DT, Poehls KA.  1980.  A Compensation Mechanism for the Central Pacific. Journal of Geophysical Research. 85:3751-3758.   10.1029/JB085iB07p03751   AbstractWebsite

Geos 3 derived geoid heights and sea floor topography were averaged into 256 square areas (203 km on a side) for a region in the central Pacific containing a large portion of the Hawaiian Island chain. The whole region is about 500 m shallower than normal sea floor of the same age. The major portion of the depth anomaly is the Hawaiian swell. Data were analyzed using a two-dimensional fast Fourier transform. A transfer function was computed to determine the part of the observed geoid height that is coherent and in phase with the topography. A number of compensation models were tested against this function. Of these models no single physically reasonable model was found to have an acceptable fit. Accordingly, two models were introduced, one compensating short-wavelength topography at a shallow depth (14 km) and the other compensating the longer wavelengths by a deep mechanism. Acceptable deep compensation models include Airy-Heiskanen type compensation at depths between 40 and 80 km. Using the transition wavelength between the two models (1100 km), an estimate is made of the amplitude and shape of the heat anomaly needed to uplift the Hawaiian swell. The peak of the anomaly has an amplitude of 530 mW m−2 and is located 275 km east of Hawaii.

Sandwell, D, Schubert G.  1980.  Geoid Height Versus Age for Symmetric Spreading Ridges. Journal of Geophysical Research. 85:7235-7241.   10.1029/JB085iB12p07235   AbstractWebsite

Geoid height-age relations have been extracted from Geos 3 altimeter data for large areas in the North Atlantic, South Atlantic, southeast Indian, and southeast Pacific oceans. Except for the southeast Pacific area, geoid height decreases approximately linearly with the age of the ocean floor for ages less than about 80 m.y. in agreement with the prediction of an isostatically compensated thermal boundary layer model (Haxby and Turcotte, 1978). The geoid-age data for 0 to 80 m.y. are consistent with constant slopes of −0.094±0.025, −0.131±0.041, and −0.149±0.028 m/m.y. for the South Atlantic, southeast Indian, and North Atlantic regions, respectively. For ages greater than 80 m.y. the geoid-age relation for the North Atlantic is nearly flat, indicating a reduction in the rate of boundary layer thickening with age. The uncertainties in the geoid slope-age estimates are positively correlated with spreading velocity.

1981
Sandwell, DT.  1981.  Spreading Ridges, Fractures Zones, and Thermal Swells. Ph. D.:214., Los Angeles: University of California, Los Angeles Abstract
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1982
Sandwell, DT, Schubert G.  1982.  Geoid Height-Age Relation from Seasat Altimeter Profiles across the Mendocino Fracture-Zone. Journal of Geophysical Research. 87:3949-3958.   10.1029/JB087iB05p03949   AbstractWebsite

Twenty-eight SEASAT altimeter profiles crossing the Mendocino Fracture Zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age, between ages of 15 and 135 m.y. An unbiased estimate of the overall geoid offset along each profile is determined from a least-squares fit of the along-track derivative of the geoid to the geoid slope predicted from a simple two-layer gravitational edge effect model. Uncertainties based upon the statistical properties of each profile are also determined. A geoid slope-age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets. The results are in agreement with geoid slope-age relations determined from symmetrically spreading ridges (Sandwell and Schubert, 1980). However, the fracture zone estimates have smaller uncertainties and show less scatter. A comparison of these results with the geoid slope-age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age (20–40 m.y.). A plate cooling model with a thickness of 125 km is most compatible with the geoid slope-age estimates, although significant deviations occur; these may indicate that the lithospheric thermal structure is not entirely age dependent.

Sandwell, D, Schubert G.  1982.  Lithospheric Flexure at Fracture-Zones. Journal of Geophysical Research. 87:4657-4667.   10.1029/JB087iB06p04657   AbstractWebsite

Bathymetric profiles across six major fracture zones (FZ's) in the North Pacific are used to demonstrate the absence of vertical slip on the fossil fault planes. The scarp heights on these FZ's are constant with age and equal to the initial vertical offsets at the ridge-transform fault-FZ intersections. Because of the frozen-in scarp and the differential subsidence of lithosphere far from the FZ, the lithosphere bends in the vicinity of the FZ. This flexure results in a characteristic ridge-trough topographic FZ signature. The flexural amplitude, which is the difference between the scarp height and the overall change in depth across the FZ, increases with age. Good fits to the bathymetric profiles across the Mendocino and Pioneer FZ's are obtained by modelling the topography as the flexure of a thin elastic plate with an age-dependent effective elastic thickness. Results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450°C isotherm. Maximum bending stresses at FZ's are on the order of 100 MPa, substantially less than the stresses encountered at subduction zones. Because the Mendocino and Pioneer FZ's are separated by less than a flexural wavelength, they are elastically coupled.

Liu, CS, Sandwell DT, Curray JR.  1982.  The Negative Gravity-Field Over the 85-Degrees-E Ridge. Journal of Geophysical Research. 87:7673-7686.   10.1029/JB087iB09p07673   AbstractWebsite

An isopach map made from seismic reflection and refraction data in the Bay of Bengal shows two prominent N-S trending features in the basement topography. One is the northernmost portion of the Ninetyeast Ridge which is totally buried by sediments north of 10°N. The other buried ridge trends roughly N-S for 1400 km at 85°E to the latitude of Sri Lanka and then curves toward the west. It has basement relief up to 6 km. Two free-air gravity anomaly profiles across the region show a strong gravity low (∼−60 mGal) over the 85°E Ridge, while the Ninetyeast Ridge shows a gravity high. To understand the negative free-air gravity anomaly over the 85°E Ridge, we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load. A plausible formation history for a buried ridge consists of at least two major episodes. The first is the formation of the ridge on a lithosphere with a flexural rigidity of D1. At some later time the ridge is buried by an influx of sediments, the lithosphere is cooler, and the flexural rigidity has increased to D2. The character of the gravity field depends primarily upon the initial and final values of flexural rigidity. These D1 and D2 values are varied to obtain good agreement between the model and observed gravity anomalies. Best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial. An approximate relationship between flexural rigidity and crustal age shows that the 85°E Ridge was formed on relatively young lithosphere, 5–15 m.y. old and that it was buried when the lithosphere was 40–80 m.y. old.

Sandwell, DT.  1982.  Thermal isostasy; response of a moving lithosphere to a distributed heat source. Journal of Geophysical Research. 87:1001-1014., Washington, DC, United States (USA): American Geophysical Union, Washington, DC   10.1029/JB087iB02p01001   AbstractWebsite

Spreading ridges and hot spot swells are identified by their high surface heat flow, shallow seafloor, and high geopotential. To understand these and other thermotectonic features, the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three-dimensional, time-independent heat source. The heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes. The Fourier transforms of three Green's functions (response functions), which relate the three observable fields to their common heat source, are determined analytically. Each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source. However, the ratio of the gravity response function to the topography response function (i.e., gravity/topography transfer function) is nearly isotropic and has a maximum lying between the flexural wavelength and 2pi times the thickness of the thermal boundary layer. The response functions are most useful for determining the surface heat flow, seafloor topography, and geopotential for complex lithospheric thermal structures. In practice, these three observables are calculated by multiplying the Fourier transform of the heat source by the appropriate response function and inverse transforming the products. Almost any time-independent thermotectonic feature can be modeled using this technique. Included in this report are examples of spreading ridges and thermal swells, although more complex geometries such as ridges offset by transform faults and RRR-type triple junctions can also be modeled. Because forward modeling is both linear and computationally simple, the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields.

1984
Sandwell, DT.  1984.  Along-track deflection of the vertical from Seasat : GEBCO overlays. , Rockville, Md.: U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, National Geodetic Survey, Charting and Geodetic Services : For sale by the National Geodetic Information Center Abstract
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Cheney, RE, Douglas BC, Sandwell DT, Marsh JG, Martin TV.  1984.  Applications of Satellite Altimetry to Oceanography and Geophysics. Marine Geophysical Researches. 7:17-32.   10.1007/bf00305408   AbstractWebsite

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.

Sandwell, DT.  1984.  A Detailed View of the South-Pacific Geoid from Satellite Altimetry. Journal of Geophysical Research. 89:1089-1104.   10.1029/JB089iB02p01089   AbstractWebsite

Images of sea surface undulations in the South Pacific have been constructed from GEOS 3 and SEASAT altimeter data. Height discrepancies at crossover points, associated with long-wavelength radial orbit error, were suppressed by taking along-track derivatives of the ascending and descending profiles. These geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical. Finally, the results are displayed by using the hill shading technique, where gray-tone images represent the innner product of the deflection vector with an assigned sun vector. Less apparent sea surface undulations can be enhanced by varying the sun's zenith and azimuth. Shorterwavelength sea surface undulations reflect seafloor topography. For instance, fracture zones (FZ's) appear as elongated sharp steps in the sea surface, while seamounts produce circular bumps. Since large areas of the South Pacific are unsurveyed, many previously undetected features appear on the images. Comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to Easter Island's expression. The dominant features in the images, however, are the large age-offset FZ's such as the Eltanin and Udintsev FZ's. The images reveal that the Eltanin FZ is connected to the Louisville Ridge; combined they produce a continuous geoid signature across most of the South Pacific. This supports the hypothesis of Hayes and Ewing (1968) that the Louisville Ridge is the northwest extension of the Eltanin FZ.

Keating, B, Cherkis NZ, Fell PW, Handschmacher D, Hey RN, Lazarewicz A, Naar DF, Perry RK, Sandwell D, Schwank DC, Vogt P, Zondek B.  1984.  Field-Tests of Seasat Bathymetric Detections. Marine Geophysical Researches. 7:69-71.   10.1007/bf00305411   AbstractWebsite

Knowledge of the locations and sizes of seamounts is of great importance in applications such as inertial navigation and ocean mining. The quality and density of bathymetry data in the equatorial regions and the southern hemisphere are, unifortunately, highly variable. Our present knowledge of bathymetry, and in particular of seamount locations and characteristics, is based upon ship surveys, which are both time-consuming and expensive. It is likely that a significant number of uncharted seamounts exist throughout the oceans, and remote-sensing techniques may be the most effective means of locating them.

Wagner, CA, Sandwell DT.  1984.  The Gravsat Signal over Tectonic Features. Journal of Geophysical Research. 89:4419-4426.   10.1029/JB089iB06p04419   AbstractWebsite

The range rate between two close gravitational satellites (GRAVSAT) in low earth orbit has been evaluated over model tectonic features such as mountains and ranges, fracture zones, and trenches. Models are locally compensated and consist of both point mass dipoles and sheet mass dipoles. Masses and depths of compensation are chosen to approximate known gravity signatures. The results show that for two satellites at 160 km altitude with 3° separation, significant signal power (>1 μm/s) remains for most extended features at wavelengths less than 200 km. Furthermore, there is strong sensitivity in the signal from these features to lateral and vertical changes of the order of 1 km and less. In addition, the signal of hidden geologic structures such as dikes, salt domes, and ore bodies may also stand above 1 μm/s for this low orbiting pair. Thus, it may prove to be efficient to model the high-frequency GRAVSAT signal directly in terms of the parameters of tectonic-topographic features and their compensation.

Douglas, BC, Agreen RW, Sandwell DT.  1984.  Observing Global Ocean Circulation with Seasat Altimeter Data. Marine Geodesy. 8:67-83. AbstractWebsite
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Sandwell, DT, Agreen RW.  1984.  Seasonal-Variation in Wind-Speed and Sea State from Global Satellite Measurements. Journal of Geophysical Research-Oceans. 89:2041-2051.   10.1029/JC089iC02p02041   AbstractWebsite

The GEOS 3 altimeter, which collected data intermittently for nearly 4 years, has measured significant wave heights and surface wind speeds over most of the world's oceans. Using these data, we have constructed contour maps of spatial variations in sea state and wind speed for winter and summer. To obtain reliable averages in the southern oceans, we low-pass filtered the data using a two-dimensional Gaussian filter with a half width of 600 km. The wind speed maps show that the zonal surface wind patterns, such as the westerlies, the horse latitudes, the trade winds, and the doldrums, shift south by about 10° between winter and summer. As expected, the highest wind speeds and sea states occur during the winter months in the mid-latitudes, 30°–60°. The most striking feature of the maps, however, is the large asymmetry in the summer to winter variation between the two hemispheres. The largest seasonal variations in sea state and wind speed occur in the northern hemisphere oceans and especially in the North Atlantic, where there is almost a factor of 2 variation. In contrast, the summer to winter variation in wind speed and sea state in the southern hemisphere oceans is relatively small. For example, the summer to winter increase in wind speed at 50°S is less than 10%, while at 50°N it is more than 50%. This differing variability can be attributed to the asymmetric distribution of continental area between the two hemispheres and the low effective heat capacity of the continents relative to the oceans.

Sandwell, DT.  1984.  Thermomechanical Evolution of Oceanic Fracture-Zones. Journal of Geophysical Research. 89:1401-1413.   10.1029/JB089iB13p11401   AbstractWebsite

A fracture zone (FZ) model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere. As the lithosphere cools by conduction, thermal and mechanical boundary layers develop and increase in thickness as (age)1/2. Surface expressions of this development, such as topography and deflection of the vertical (i.e., gravity field), are most apparent along major FZ's because of the sharp age contrast. A simple model, including the effects of lateral heat transport but with no elastic layer, predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the FZ ages. Observations, however, show that both the FZ topography and the deflection of the vertical remain sharp as the FZ evolves. These two observations, as well as the observed asymmetry in deflection of the vertical profiles across the Udintsev, Romanche, and Mendocino FZ's, are explained by including a continuous elastic layer in the model. The asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the FZ. Modeling also shows that the evolution of the FZ topography is extremely sensitive to the initial thermal structure near the ridge-transform intersection. Model geoid steps and their development with age are used to access techniques for measuring geoid offsets across FZ's. Reasonable step estimation techniques will underestimate the overall step amplitude by up to 50%. This implies that abnormally thin thermal boundary layers, derived from studies of geoid height versus age, are not required by the data.

1985
McAdoo, DC, Sandwell DT.  1985.  Folding of Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 90:8563-8569.   10.1029/JB090iB10p08563   AbstractWebsite

Folding of the lithosphere just south of the Bay of Bengal appears as (1) undulations in acoustic basement topography and (2) as linear geoid undulations in the Seasat altimeter data. From the Seasat data we find that the east-west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km. The horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mGal. Elastic models of oceanic lithosphere have, in the past, been used to demonstrate the implausibility of lithosphere buckling, or folding, in response to compression. These elastic models typically predict that compressive stresses of about 5 GPa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern Indian Ocean (40–70 Ma). These stresses exceed the strength of lithospheric rock. We use an elastic-plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12% of the elastic buckling stress. We further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km, i.e., the wavelength observed from Seasat. Our results reinforce earlier speculation that this folding may be related to the Himalayan orogeny.

Sandwell, DT, Agreen RW.  1985.  Seasonal-Variation in Wind-Speed and Sea State from Global Satellite Measurements - Reply. Journal of Geophysical Research-Oceans. 90:5009-5010.   10.1029/JC090iC03p05009   AbstractWebsite
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1986
Cheney, RE, Douglas BC, McAdoo DC, Sandwell DT.  1986.  Geodetic and oceanographic applications of satellite altimetry. Space geodesy and geodynamics. ( Anderson A, Cazenave A, Eds.)., London, United Kingdom (GBR): Academic Press, London AbstractWebsite
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Sandwell, DT, Milbert DG, Douglas BC.  1986.  Global Nondynamic Orbit Improvement for Altimetric Satellites. Journal of Geophysical Research-Solid Earth and Planets. 91:9447-9451.   10.1029/JB091iB09p09447   AbstractWebsite

The largest source of error in satellite altimetry is in the radial position of the satellite. Radial orbit errors of more than a few decimeters prohibit basin-scale studies of sea surface height variability. We explore nondynamic techniques for reducing this error. Sea surface height differences at intersections of satellite altimeter profiles (crossover data) provide a strong constraint on radial orbit error but do not uniquely define it. The portion of orbit error that is a function of latitude and longitude only produces no crossover differences and therefore cannot be recovered with crossover data. Using mathematics (inclination functions) originally developed for satellite dynamics, we determine the entire class of orbit error functions not recoverable with crossover data. These functions are mappings of surface spherical harmonics into the orbit plane. For example, the l = 1, m = 0 surface harmonic maps into sinusoidal orbit error with a frequency of once per orbit. Nonzonal harmonics map into linear combinations of three or more frequencies that are linked by the inclination functions. Between frequencies of 0 and 2.2 cycles per orbit there are nine orbit error components that cannot be recovered using crossover data. These components are uniquely defined, however, by nine globally distributed radial tracking points. Fewer tracking points are sufficient if a smoothness criteria is applied to the orbit correction curve. Our findings suggest that radial orbit error can be significantly reduced by including a few globally distributed radar reflectors (or transponders) in the tracking network.

Mammerickx, J, Sandwell D.  1986.  Rifting of Old Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 91:1975-1988.   10.1029/JB091iB02p01975   AbstractWebsite

Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (>400 km) spreading center jumps have occurred in the past. The present-day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the weakened lithosphere (during rifting, the crack is filled from above by normal faulting and wedge subsidence; viscous upwelling fills the crack from below), (3) spreading at the rift site results in a ridge bounded by two troughs (spreading ceases at the dying spreading center, resulting in a deep central graben surrounded by flexural ridges; periods of slow spreading at both spreading centers produce rough topography), (4) ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site. The new spreading center develops into a normal spreading rift. The major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site. The proximal trough (nearer the new spreading ridge) and the distal trough (farther from the new ridge) are first-order topographic features, 100–1000 km long and 300 km wide, resembling fracture zones with which they are often confused. They share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages, but unlike fracture zones they are generally parallel to magnetic lineations.

Detrick, RS, Von Herzen RP, Parsons B, Sandwell D, Dougherty M.  1986.  Heat-Flow Observations on the Bermuda Rise and Thermal Models of Midplate Swells. Journal of Geophysical Research-Solid Earth and Planets. 91:3701-3723.   10.1029/JB091iB03p03701   AbstractWebsite

The Bermuda Rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western North Atlantic. The magnitudes of the depth and geoid anomalies associated with the Bermuda Rise are similar to the anomalies associated with other swells surrounding recent volcanic islands (e.g., Hawaii), suggesting that despite the lack of recent volcanism on Bermuda, the rise has a similar origin to other midplate swells. Results are reported from 171 new heat flow measurements at seven carefully selected sites on the Bermuda Rise and the surrounding seafloor. Off the Bermuda Rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict, with the measured heat flow apparently reaching a uniform value of about 50 mW m−2 on 120 m.y. old crust. On the Bermuda Rise the heat flow is significantly higher (57.4±2.6 mW m−2) than off the swell (49.5±1.7 mW m−2). The magnitude of the anomalous heat flux (8–10 mW m−2) is comparable to that previously found along the older portion of the Hawaiian Swell near Midway. The existence of higher heat flow on both the Hawaiian Swell and Bermuda Rise indicates that these features fundamentally have a thermal origin. The differences in the shape, uplift, and subsidence histories of the Hawaiian Swell and Bermuda Rise can be quantitatively explained by the different absolute velocities of the Pacific and North American plates moving across a distributed heat source in the underlying mantle. Two-dimensional numerical convection models indicate that the observed depth, geoid, and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection.

Sandwell, DT.  1986.  Thermal-Stress and the Spacings of Transform Faults. Journal of Geophysical Research-Solid Earth and Planets. 91:6405-6417.   10.1029/JB091iB06p06405   AbstractWebsite

Bathymetric charts are used with satellite altimeter profiles to locate major ridge-transform intersections along five spreading ridges. The ridges are the Mid-Atlantic Ridge, the East Pacific Rise, the Chile Rise, the Pacific-Antarctic Rise, and the Southeast Indian Ridge. Analysis of these data show spacings between transform faults W increase linearly with spreading rate ν (W/ν = 6.28 m.y.). This linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate ν. The traction-free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults. A two-dimensional thermoelastic solution for the in-plane stress shows the largest stress component is tensional and parallel to the ridge. Stresses are zero at the ridge and increase as (age)½ to a maximum value at an age of W/4ν. All stress components are small for ages greater than W/ν. When the transform spacing is large compared with the spreading rate (W/ν > 100 m.y.) thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 Ma. The observed maximum ratio of transform spacing to spreading rate (W/ν = 10 m.y.) results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 Ma. Thus transform faults relieve most of the thermal stress. Model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge. Moreover, oceanic intraplate earthquakes rarely occur in lithosphere older than 30 Ma as predicted by the model. These and other geophysical observations confirm Turcotte's hypothesis that transform faults are thermal contraction cracks.

1987
Sandwell, DT.  1987.  Biharmonic Spline Interpolation of Geos-3 and Seasat Altimeter Data. Geophysical Research Letters. 14:139-142.   10.1029/GL014i002p00139   AbstractWebsite

Green functions of the biharmonic operator, in one and two dimensions, are used for minimum curvature interpolation of irregularly spaced data points. The interpolating curve (or surface) is a linear combination of Green functions centered at each data point. The amplitudes of the Green functions are found by solving a linear system of equations. In one (or two) dimensions this technique is equivalent to cubic spline (or bicubic spline) interpolation while in three dimension it corresponds to multiquadric interpolation. Although this new technique is relatively slow, it is more flexible than the spline method since both slopes and values can be used to find a surface. Moreover, noisy data can be fit in a least squares sense by reducing the number of model parameters. These properties are well suited for interpolating irregularly spaced satellite altimeter profiles. The long wavelength radial orbit error is suppressed by differentiating each profile. The shorter wavelength noise is reduced by the least squares fit to nearby profiles. Using this technique with 0.5 million GEOS-3 and SEASAT data points, it was found that the marine geoid of the Caribbean area is highly correlated with the sea floor topography. This suggests that similar applications, in more remote, areas may reveal new features of the sea floor.

Winterer, EL, Sandwell DT.  1987.  Evidence from EN-Echelon Cross-Grain Ridges for Tensional Cracks in the Pacific Plate. Nature. 329:534-537.   10.1038/329534a0   AbstractWebsite

Sea-floor topography in the Pacific is mainly aligned with original spreading directions1, but is overprinted by alignments created by mid-plate processes. Spreading produces abyssal hills and fracture zones, and mid-plate volcanism generates seamounts, isolated or in chains. A different category of topography, the 'Cross-grain', discovered in geoid-height data collected by the Seasat radar altimeter2, comprises linear troughs and swells spaced ~200 km apart, oblique to fracture zones and abyssal hills but parallel to the Hawaiian chain. Three models have been proposed for the Cross-grain: small-scale convection, organized into longitudinal rolls by the shear of the Pacific Plate2; compressive buckling3; and lithospheric boudinage resulting from plate-wide tensile stresses4,5. None of the previously available data ruled out any of these models. Here we report multi-beam bathymetric data revealing long, narrow en-echelon ridges along the Cross-grain, interpreted as evidence of tension cracks in the Pacific plate.

1988
Sandwell, DT, Renkin ML.  1988.  Compensation of Swells and Plateaus in the North Pacific - No Direct Evidence for Mantle Convection. Journal of Geophysical Research-Solid Earth and Planets. 93:2775-2783.   10.1029/JB093iB04p02775   AbstractWebsite

At intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation. A low geoid/topography ratio (<2 m/km) signifies shallow Airy compensation. A higher ratio (∼6 m/km) signifies thermal isostasy and/or dynamic uplift from a mantle plume. A very high geoid/topography ratio (>8 m/km) in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection. After subtracting a reference geoid from the observed geoid, previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000–4000 km. Since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid/topography ratios (10–20 m/km), they were believed to reflect the planform of mantle convection. We show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10. Since the geoid spectrum is “red,” the residual geoid is dominated by degree 11. When the harmonics of the reference geoid are rolled off gradually, the regular pattern of geoid highs and lows disappears. In the Northeast Pacific, the new residual geoid reflects the lithosphere age offsets across the major fracture zones. In the Northwest Pacific, the residual geoid corresponds to isostatically compensated swells and plateaus. We have calculated the geoid/topography ratio for 10 swells and plateaus and have found a range of compensation depths. The highest geoidAopography ratio of 5.5 m/km occurs on the flanks of the Hawaiian Swell. Intermediate ratios occur in four areas, including the Midway Swell. These intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of Airy-compensated seamounts. Low geoid/topography ratios occur over the remaining five areas (e.g., Emperor Seamounts), reflecting the absence of a thermal swell. Our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid. We see only indirect evidence of thermal plumes reheating the lower lithosphere.