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.