Publications

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1984
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.

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.

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.

1988
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, 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.

1989
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.

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.

1990
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
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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.)

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.

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.

1991
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.

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.

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

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

1992
McKenzie, D, Ford PG, Johnson C, Parsons B, Sandwell D, Saunders S, Solomon SC.  1992.  Features on Venus Generated by Plate Boundary Processes. Journal of Geophysical Research-Planets. 97:13533-13544.   10.1029/92JE01350   AbstractWebsite

Various observations suggest that there are processes on Venus that produce features similar to those associated with plate boundaries on Earth. Synthetic aperture radar images of Venus, taken with a radar whose wavelength is 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound source whose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis region of Venus but are not clearly visible elsewhere. The composition of the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraft corresponds to that expected from adiabatic decompression, like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of Earth can also be applied to Venus.

1993
Atwater, T, Sclater J, Sandwell D, Severinghaus J, Marlow M.  1993.  Fracture zone traces across the North Pacific Cretaceous Quiet Zone and their tectonic implications. The Mesozoic Pacific : geology, tectonics, and volcanism : a volume in memory of Sy Schlanger. ( Pringle MS, Sager WW, Sliter WV, Stein S, Eds.).:137-154., Washington, DC: American Geophysical Union Abstract
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1994
Schubert, G, Moore WB, Sandwell DT.  1994.  Gravity over Coronae and Chasmata on Venus. Icarus. 112:130-146.   10.1006/icar.1994.1174   AbstractWebsite

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km(-1). The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km(-1). The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (approximate to 100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata. (C) 1994 Academic Press, Inc.

1995
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.

1997
Phiilips, RJ, Johnson CL, Mackwell SJ, Morgan P, Sandwell DT, Zuber MT.  1997.  Lithospheric Mechanics and Dynamics of Venus. Venus II--geology, geophysics, atmosphere, and solar wind environment. ( Bougher SW, Hunten DM, Phillips RJ, Eds.)., Tucson, Ariz.: University of Arizona Press Abstract
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2000
Maia, M, Ackermand D, Dehghani GA, Gente P, Hekinian R, Naar D, O'Connor J, Perrot K, Morgan JP, Ramillien G, Revillon S, Sabetian A, Sandwell D, Stoffers P.  2000.  The Pacific-Antarctic Ridge-Foundation hotspot interaction: a case study of a ridge approaching a hotspot. Marine Geology. 167:61-84.   10.1016/s0025-3227(00)00023-2   AbstractWebsite

The Foundation hotspot-Pacific-Antarctic Ridge (PAI) system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it. The morphology, crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the PAR show evidence of the influence of the hotspot, that is presently located roughly 35 km west of the spreading ridge axis. Along-axis variation in the Mantle Bouguer anomaly is about 28 mGal, corresponding to a crustal thickening of 1.5 km where the hotspot is nearer to the PAR. Anomalous ridge elevation is 650 m and the along-axis width of the chemical anomaly is 200 km. A comparison of these axial parameters with those derived for other ridge-hotspot systems, suggests that the amount of plume material reaching the ridge axis is smaller for the Foundation-PAR system. This implies a weaker connection between the plume and the ridge. Cumulative effects of a fast spreading rate and of a fast ridge-hotspot relative motion can be responsible for this weak plume-ridge flow. The how from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot. (C) 2000 Elsevier Science B.V. All rights reserved.

Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

2002
Mellors, RJ, Sichoix L, Sandwell DT.  2002.  Lack of precursory slip to the 1999 Hector Mine, California, earthquake as constrained by InSAR. Bulletin of the Seismological Society of America. 92:1443-1449.   10.1785/0120010244   AbstractWebsite

We looked for evidence of interseismic strain occurring between the 1992 Landers earthquake and the 1999 Hector Mine earthquake near the Lavic Lake and Bullion faults by using interferometric synthetic aperture radar (InSAR). Interferograms covering the Hector Mine epicentral region were studied for possible slip along the Bullion and Lavic Lake faults by both visual inspection and a matched filter technique intended to emphasize slip located at the nucleation point. Some indications of possible deformation associated with the 5 July 1992 M-L 5.4 Pisgah event was observed, but high decorrelation prevented a conclusive determination. We have seen no evidence for precursory slip in the epicentral region up to 30 days before the Hector Mine event. We estimated that the slip equivalent to a M-w 4.5 event would have been observable in the months before the Hector Mine event, and this places an upper bound on the long-term precursory slip, had it occurred. We have noted that InSAR is well suited for detecting precursory slip in general due to the high spatial resolution and the lack of ground instrumentation required but that the detection level depends on the depth and orientation of the slip.

Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.