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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.

Mueller, D, Sandwell DT, Tucholke BE, Sclater JG, Shaw PR.  1991.  Depth to basement and geoid expression of the Kane Fracture Zone: A comparison. Marine Geophysical Researches. 13:105-129. AbstractWebsite

Geoid data from Geosat and subsatellite basement depth profiles of the Kane Fracture Zone in the central North Atlantic were used to examine the correlation between the short-wavelength geoid ( lambda = 25-100 km) and the uncompensated basement topography. The processing technique we apply allows the stacking of geoid profiles, although each repeat cycle has an unknown long-wavelength bias. We first formed the derivative of individual profiles, stacked up to 22 repeat cycles, and then integrated the average-slope profile to reconstruct the geoid height. The stacked, filtered geoid profiles have a noise level of about 7 mm in geoid height. The subsatellite basement topography was obtained from a recent compilation of structure contours on basement along the entire length of the Kane Fracture Zone.

Sandwell, DT.  1991.  Geophysical Applications of Satellite Altimetry. Reviews of Geophysics. 29:132-137. AbstractWebsite
Shum, CK, Werner RA, Sandwell DT, Zhang BH, Nerem RS, Tapley BD.  1990.  Variations of Global Mesoscale Eddy Energy Observed from GEOSAT. Journal of Geophysical Research-Oceans. 95:17865-&.   10.1029/JC095iC10p17865   AbstractWebsite

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2-year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty-four 17-day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm^2/s^2 for most of the western boundary currents; however, it only reaches approximately 500 cm^2/s^2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm^2/s^2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.

Sandwell, DT, McAdoo DC.  1990.  High-Accuracy, High-Resolution Gravity Profiles from 2 Years of the Geosat Exact Repeat Mission. Journal of Geophysical Research-Oceans. 95:3049-3060.   10.1029/JC095iC03p03049   AbstractWebsite

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17-day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short-wavelength altimeter noise and longer-wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross-spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal-to-noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short-wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half-wavelength resolution of 12 km by a 4.5-year satellite altimeter mapping mission.

Mayes, CL, Lawver LA, Sandwell DT.  1990.  Tectonic History and New Isochron Chart of the South-Pacific. Journal of Geophysical Research-Solid Earth and Planets. 95:8543-8567.   10.1029/JB095iB06p08543   AbstractWebsite

We have developed an internally consistent isochron chart and a tectonic history of the South Pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data. Highly accurate, vertical deflection profiles (1–2 μrad), derived from 22 repeat cycles of Geosat altimetry, reveal subtle lineations in the gravity field associated with the South Pacific fracture zones. These fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas. The high density of Geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates. Magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map. The tectonic fabric was then used to derive new poles of rotation for 12 selected times in the Late Cretaceous and Cenozoic. From our reconstructions, we estimated the former location of the spreading centers in order to derive a new set of isochrons (interpreted unes of equal age on the ocean floor). We believe that the use of new Geosat altimeter data in combination with a multi-plate reconstruction has led to an improvement in our understanding of South Pacific tectonics.

Koeberl, C, Sharpton VL, Harrison MT, Sandwell D, Murali AV, Burke K.  1990.  The Kara/Ust-Kara twin impact structure; a large-scale impact event in the Late Cretaceous. Special Paper - Geological Society of America. 247( Sharpton VL, Ward PD, Eds.).:233-238., Boulder, CO, United States (USA): Geological Society of America (GSA), Boulder, CO AbstractWebsite
Royer, J-Y, Gahagan LM, Lawver LA, Mayes CL, Nuernberg D, Sandwell DT, Scotese CR.  1990.  A tectonic chart for the Southern Ocean derived from Geosat altimetry data. AAPG Studies in Geology. 31( St. John B, Ed.).:89-99., Tulsa, OK, United States (USA): American Association of Petroleum Geologists, Tulsa, OK AbstractWebsite

Presented is a new tectonic fabric map of the southern ocean south of 45S, derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications. (Auth.)

Royer, JY, Sandwell DT.  1989.  Evolution of the Eastern Indian-Ocean since the Late Cretaceous - Constraints from Geosat Altimetry. Journal of Geophysical Research-Solid Earth and Planets. 94:13755-13782.   10.1029/JB094iB10p13755   AbstractWebsite

We propose a new model for the tectonic evolution of the eastern Indian Ocean from the Late Cretaceous to the present. Two types of data are used to improve previously published reconstructions. First, recent reinterpretations of seafloor magnetic anomalies, between Australia and Antarctica and in the Wharton Basin, provide new constraints on spreading rates and the timing of major reorganizations. Second, vertical deflection profiles (i.e., horizontal gravity anomaly), derived from 22 repeat cycles of Geosat altimeter data, reveal the tectonic fabric associated with fracture zones. These new Geosat data provide tight constraints on paleospreading directions. For example, three prominent fracture zones can be traced from south of Tasmania to the George V Basin, Antarctica, providing an important constraint on the relative motions of Australia and Antarctica through the Late Eocene. In addition, the Geosat profiles are used to locate the conjugate continental margins and continent-ocean boundaries of Australia and Antarctica, as well as the conjugate rifted margins of Kerguelen Plateau and Broken Ridge. Based on a compilation of magnetic anomaly data from the Crozet Basin, the Central Indian Basin, the Wharton Basin and the Australian-Antarctic Basin, ten plate tectonic reconstructions are proposed. Reconstructions at chrons 5 (11 Ma), 6 (21 Ma), 13 (36 Ma) and 18 (43 Ma) confirm that the Southeast Indian Ridge behaved as a single plate boundary since chron 18. The constraints from the Geosat data provide an improvement in the fit of the Kerguelen Plateau and Broken Ridge at chron 20 (46 Ma). To avoid overlaps between Broken Ridge and the Kerguelen Plateau prior to their breakup, our model includes relative motions between the northern and southern provinces of the Kerguelen Plateau. Finally, we examine the implications of our model for the relative motions of India, Australia and Antarctica on the tectonic evolution of the Kerguelen Plateau and Broken Ridge, and the adjacent Labuan Basin and Diamantina Zone, as well as the emplacement of the Ninetyeast Ridge and the Kerguelen Plateau over a fixed hot spot.

Schubert, G, Sandwell D.  1989.  Crustal Volumes of the Continents and of Oceanic and Continental Submarine Plateaus. Earth and Planetary Science Letters. 92:234-246.   10.1016/0012-821x(89)90049-6   AbstractWebsite

Global topographic data and the assumption of Airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus. The calculated crustal volumes are 7182 × 10^6 km^3 for the continents, 242 × 10^6 km^3 for continental submarine plateaus, and 369 × 10^6 km^3 for oceanic plateaus. The Falkland Plateau and the Lord Howe Rise are the two largest continental submarine plateaus with volumes of 48 × 10^6 km^3 and 47 × 10^6 km^3, respectively. Total continental crustal volume is 7581 × 10^6 km^3 (including the volume of continental sediments on the ocean floor 160 × 10^6 km^3), in good agreement with previous estimates. Continental submarine plateaus on the seafloor comprise 3.2% of the total continental crustal volume. The largest oceanic plateaus in order of decreasing size are the Ontong-Java Plateau, the Kerguelen Plateau, the Caribbean, the Chagos Laccadive Ridge, the Ninetyeast Ridge, and the Mid-Pacific Mountains. Together they comprise 54% of the total anomalous crustal volume in oceanic plateaus. An upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3.7 km^3/yr, a value that assumes accretion of all oceanic plateaus, with a total volume of 4.9% of the continental crustal volume, on a 100 Myr time scale. Even if a substantial fraction of the crustal volume in oceanic plateaus is subducted, accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale gives an upper bound to the continental crustal subtraction rate of 2.4 km^3/yr, much larger than recent estimates of crustal subtraction by subduction of seafloor sediments. Effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus. A potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents, entrapment of the continental fragments in the seafloor, and subduction of the fragments with the oceanic lithosphere. This process may be occurring in the Mediterranean for Corsica and Sardinia.

Sandwell, DT, Mackenzie KR.  1989.  Geoid Height Versus Topography for Oceanic Plateaus and Swells. Journal of Geophysical Research-Solid Earth and Planets. 94:7403-7418.   10.1029/JB094iB06p07403   AbstractWebsite

Oceanic plateaus and swells are a major component of the seafloor topography, yet they remain among the most poorly understood features. This is especially true of the oceanic plateaus which show large variations in crustal thickness. To determine the depth and mode of compensation for 53 of the largest plateaus and swells, we analyzed the relationship between geoid height and topography in polygonal areas containing each feature. Both geoid height and topography were first band-pass filtered (400 km < l < 4000 km) to isolate the signal associated with local compensation from flexural and deep mantle signals. The ratio of geoid height to topography was then determined by fitting a straight line to the data. Except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with Airy and thermal compensation models. Eighteen features have high geoid/topography ratios that cannot be explained by the Airy compensation model of crustal thickening. These features (thermal swells) are partially supported by thermal buoyancy forces in the lower half of the lithosphere. The ratios are highest for active hot spot swells and decay, with the thermal age of the swell, to values consistent with Airy compensation of the enduring volcanic edifice. The remaining features (plateaus) have lower geoid/topography ratios in agreement with the Airy compensation model. Those plateaus with average height greater than 4 km are thought to be continental fragments; the shorter plateaus tend to be volcanic features. Modified continental plateaus, presumably small fragments of extended and intruded continental margin crust, cluster around heights of ∼3 km, overlapping the range associated with oceanic plateaus. Since the origin of many plateaus is poorly understood, this global geoid/topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells.

McAdoo, DC, Sandwell DT.  1989.  On the Source of Cross-Grain Lineations in the Central Pacific Gravity-Field. Journal of Geophysical Research-Solid Earth and Planets. 94:9341-9352.   10.1029/JB094iB07p09341   AbstractWebsite

Subtle lineations in the marine gravity field of the central Pacific derived from Seasat altimeter data were observed by Haxby and Weissel (1986). They suggested that these “cross-grain” lineations were evidence of small-scale convection beneath the Pacific plate. We have examined these features by comparing multiple, collinear gravity and bathymetry profiles in the Fourier transform domain. Our nine gravity profiles were each obtained by stacking (averaging) three or more individual, repeat Geosat/ERM altimeter passes. Prior to stacking, the individual Geosat passes were fit to a cubic spline and then differentiated along track to produce along-track deflections of the vertical (or horizontal gravity). Corresponding bathymetric profiles were produced by projecting, onto Geosat ground tracks, bathymetric observations from six R/V Thomas Washington legs and three R/V Conrad legs that virtually coincide with these Geosat tracks. After Fourier transforming the resulting gravity and bathymetry profiles, we estimate admittances of gravity to bathymetry. These admittances are generally low; they also tend to be negative at very short wavelengths (λ<50 km). They are consistent with models of flexural isostatic compensation by a very thin lithosphere (approximately 2 km). They are not consistent with models of dynamic compensation. We suggest, therefore, that either (1) these cross-grain lineations began to form very near the East Pacific Rise or (2) they formed on older, anomalously weak lithosphere. We also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with, secondarily, loads on the seafloor. Depths of these subseafloor loads appear not to exceed significantly typical Moho depths.

Small, C, Sandwell DT.  1989.  An Abrupt Change in Ridge Axis Gravity with Spreading Rate. Journal of Geophysical Research-Solid Earth and Planets. 94:17383-17392.   10.1029/JB094iB12p17383   AbstractWebsite

The global mid-ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate. Fast spreading ridges have axial highs with little bathymetric relief and low-amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium. Slow spreading ridges have large axial valleys bounded by rugged topography (Macdonald, 1982) and large axial gravity troughs indicating that they are dynamically maintained. While this variation in ridge axis morphology with spreading rate has been observed, it has not been analyzed in a comprehensive manner. Moreover, it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate. Such observations would provide important constraints on models of ridge axis dynamics. Vertical deflection profiles collected by the Geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate. In this study, we have analyzed 44 Geosat profiles over ridges with spreading rates ranging from 14 to 155 mm/yr. In agreement with previous studies, we find that slow spreading ridges (<60 mm/yr) usually have high amplitude gravity troughs (40–100 μrad = 40–100 mGal), while fast spreading ridges (>70 mm/yr) are characterized by low-amplitude ridge axis highs (∼15 μrad). Unexpectedly, we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60–70 mm/yr. Ridge axis gravity signatures are highly variable for rates less than 65 mm/yr and very uniform at higher rates. The transition of the gravity signature appears to be more abrupt than the transition of the topographic signature, suggesting an abrupt change in the style of isostatic compensation with spreading rate. Published models of ridge axis dynamics do not explain this sharp transition.

Sandwell, DT, Zhang B.  1989.  Global Mesoscale Variability from the Geosat Exact Repeat Mission - Correlation with Ocean Depth. Journal of Geophysical Research-Oceans. 94:17971-17984.   10.1029/JC094iC12p17971   AbstractWebsite

We have developed a new technique for extracting global mesoscale variability from satellite altimeter profiles having large radial orbit error (∼3 m). Long-wavelength radial orbit error, as well as other long-wavelength errors (e.g., tides, ionospheric-atmospheric delay, and electromagnetic bias), are suppressed by taking the derivative (slope) of each altimeter profile. A low-pass filter is used to suppress the short-wavelength altimeter noise (λ<100 km). Twenty-two repeat slope profiles are then averaged to produce a mean sea surface slope profile having a precision of about 0.1 μrad. Variations in sea surface slope, which are proportional to changes in current velocity, are obtained by differencing individual profiles from the average profile. Slopes due to mesoscale dynamic topography are typically 1 μrad (i.e., a 0.1-m change in topography over a 100-km distance). Root-mean-square (rms) slope variability as low as 0.2 μrad are found in the southeast Pacific, and maximum slope variations up to 6–8 μrad are found in major western boundary currents (e.g., Gulf Stream, Kuroshio, Falkland, and Agulhas) and Antarctic Circum-polar Current (ACC) systems. The global rms variability map shows previously unknown spatial details that are highly correlated with seafloor topography. Over most areas, the rms slope variability is less than 1 μrad. However at mid-latitudes, areas of higher variability occur in deep water (>3 km) adjacent to continental shelves, spreading ridges, and oceanic plateaus. Variability is low in shallower areas (<3 km). Along the ACC, the meso-scale variability appears to be organized by the many shallow areas in its path. We do not see convincing evidence that variability is higher downstream from topographic protrusions. Instead, the areas of highest variability occur in the deep basins (>4km).

Royer, JY, Sclater JG, Sandwell DT.  1989.  A Preliminary Tectonic Fabric Chart of the Indian-Ocean. Proceedings of the Indian Academy of Sciences-Earth and Planetary Sciences. 98:7-24. AbstractWebsite
Craig, CH, Sandwell DT.  1988.  Global Distribution of Seamounts from Seasat Profiles. Journal of Geophysical Research-Solid Earth and Planets. 93:10408-10420.   10.1029/JB093iB09p10408   AbstractWebsite

Bathymetry profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins, but only a small fraction of the seafloor has been sampled by ships. At the present exploration rate it will take several centuries to map significant portions of the seafloor topography. Satellite altimetry, which maps the topography of the equipotential sea surface, is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years. Using a model of a Gaussian-shaped seamount loading a thin elastic lithosphere, we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile. The model predicts that the seamount diameter is equal to the peak-to-trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed. Moreover, the model suggests that these two measurements are relatively insensitive to the cross-track location of the seamount. We confirm these model predictions using Seasat altimeter profiles crossing 14 well surveyed seamounts in the Pacific. We then apply the measurement technique to 26 × 106 million kilometers of Seasat profiles resulting in a new global set of seamount locations. Approximately one quarter of the seamounts identified in Seasat profiles were previously uncharted. Modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid; therefore the set of locations is not a straightforward sampling of the total seamount population, but is weighted toward seamounts which are poorly compensated. A preliminary analysis indicates considerable variations in population density and type across the oceans; most notable among them are the absence of seamounts in the Atlantic, variations in population density across large age-offset fracture zones in the Pacific, the prevalence of small signatures in the Indian Ocean, and the existence of linear trends in the large seamounts of the west Pacific.

Sandwell, DT, McAdoo DC.  1988.  Marine Gravity of the Southern-Ocean and Antarctic Margin from Geosat. Journal of Geophysical Research-Solid Earth and Planets. 93:10389-&.   10.1029/JB093iB09p10389   AbstractWebsite

In November of 1986 the U.S. Navy satellite Geosat began collecting unclassified (gravity) altimeter data as part of its exact repeat mission (ERM). For national security reasons the Geosat orbit was arranged so that it closely follows the Seasat satellite altimeter ground track. However, there are two advantages of the Geosat data over the Seasat data. First, because of improvements in altimeter design, Geosat profiles are about 3 times more precise than Seasat profiles. This corresponds to an accuracy of 2–3 μrad (i.e., 2–3 mGal) for wavelengths greater than 20 km. Second, the Geosat altimeter data were collected when the Antarctic ice coverage was minimal (February 1987 to March 1987), while Seasat was only active during an Antarctic winter (June 1978 to September 1978). These new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to Antarctica. Seven large age-offset fracture zones, apparent in the Geosat data, record the early breakup of Gondwana. Finally, the new data reveal the detailed gravity signatures of the passive and active continental margins of Antarctica. These data are an important reconnaissance tool for future studies of these remote ocean areas.

Gahagan, LM, Scotese CR, Royer JY, Sandwell DT, Winn JK, Tomlins RL, Ross MI, Newman JS, Muller RD, Mayes CL, Lawver LA, Heubeck CE.  1988.  Tectonic Fabric Map of the Ocean Basins from Satellite Altimetry Data. Tectonophysics. 155:1-&.   10.1016/0040-1951(88)90258-2   AbstractWebsite

Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes.This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.

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.

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.

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.

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.

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.

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.

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.