Seasat altimetry, the North Atlantic geoid, and evaluation by shipborne subsatellite profiles

Abstract

A geological evaluation of geoid anomalies over the North Atlantic is presented with emphasis on short (40–300 km) wavelength undulations mapped by Seasat radar altimetry. Long (≥3000 km) and intermediate (300–3000 km) wavelength undulations constrain subcrustal processes and are briefly reviewed. Portions of three Seasat revolutions (revs) totaling 9300 km were followed by research ship, which measured gravity, magnetics, topography, and for a 800‐km segment also seismic reflection data, to form a ground truth data set for comparison to the altimetry. The intermediate‐wavelength geoid, the residual upon removal of a (14,14) long‐wavelength surface, correlates with plate age (slope ∼10 cm/m.y.) over Cenozoic crust, and major fracture zones appear as geoid steps. A 3.5‐ to 5‐m high over the Bermuda Rise may reflect thermal rejuvenation created within the plate 40–50 Ma. Short‐wavelength geoid anomalies, typically ±10–50 cm in the open ocean, were analyzed by high‐pass (200‐ or 400‐km cutoff) along‐track filtering and smoothing. The vertical deflection computed from the Seasat geoid compares well with one based on seismic reflection. Shipborne free air gravity, except for a higher‐frequency cutoff, correlates closely with geoid anomalies. Except near seamounts, the principal contribution to geoid anomalies is fracture zone topography. All major and many minor fracture zones (FZ's) appear as geoid lineations, particularly between the 15°20′and Kane FZ, from the ridge axis to anomaly 34. The WSW trend of FZ's between anomaly 21 and 34 is clearly expressed in the altimetry, as is the smooth geoid over mid‐Cretaceous smooth basement formed by fast spreading. Crustal ribbons, 50–150 km wide, of higher or lower geoid, parallel FZ trends. Some geoid lineations are oblique to both isochron and FZ trends; east of the Puerto Rico Trench such anomalies may reflect compressional deformation within Atlantic lithosphere.

Seasat Altimetry and the South Atlantic Geoid: 1. Spectral analysis

“…In conclusion, all wavelengths shorter than 35 km are unlikely to contain any significant information and can be filtered out. This is in agreement with the estimate of 30-50 km derived by Vogt et al [1984] from a comparison of surface gravity and altimetry data in the northwestern Atlantic.…”

“…Should we have access to detailed bathymetric data, we could hope to extract the admittance functions and to sort out the thermomechanical parameters of the various lithospheric provinces. This has been done for ship gravity and bathymetric data by McKenzie and Bowin [1976], followed by Watts [1978] Louden and Forsyth [1982], and others. In the case of Seasat data it is quite rare to have access to bathymetric data of good accuracy precisely below the satellite tracks.…”

Underthrusting at the Hjort Trench, Australian‐Pacific plate boundary: Incipient subduction?

“…Where shipboard bathymetric data are not available (corresponding to most of the mapped fracture zones), fracture zones are most readily identified in the free‐air gravity anomaly field [ Smith and Sandwell , 1997] (Figure 3). Fracture zones were mapped at local minima of the free‐air gravity anomaly data (gravity troughs) [ Vogt et al , 1984]. Minimum values are up to 30 mGal lower at interpreted fracture zones than in the adjacent crust.…”

Tectonic corridors in the south Atlantic: Evidence for long‐lived mid‐ocean ridge segmentation

“…Fortunately, subsidence rates calculated using DBDB5 were found to be quite similar to the subsidence rates using ship tracks. This is despite the fact that individual locations can display significant discrepancies between ship track bathymetry and DBDB5 bathymetry [Hayes, 1991;Smith and Wessel, 1990;Vogt et al, 1984]. This study focuses on the ship swath data for our interpretations, using the South Atlantic DBDB5 data for additional estimates of segmentation where ship track data are unavailable.…”