Free-air gravity and geoid anomalies of the North Atlantic Ocean and their tectonic implications

Abstract

A free-air gravity anomaly map of the North Atlantic Ocean has been constructed at a 10 mgal contour interval utilizing all available surface ship and pendulum gravity measurements. The map shows values more positive to the north of ~32°N than to the south and values more negative in the western North Atlantic Ocean basin than in the eastern basin. The Mid-Atlantic Ridge area and topographic highs such as volcanic islands, seamounts are associated with positive values while the rift valley and fracture zone valleys are characterized by elongated negative values. The seaward edge of the continental shelf is characterized by a belt of positive free-air gravity anomalies; negative gravity values are observed just seaward of the continental slope and rise. A total of 3032 1x1° averaged free-air gravity anomalies were determined from the surface ship gravity map. A gravimetric geoid was computed from these 1x1° averaged gravity anomalies by employing Stokes' integral theorem. Comparisons were made between the 1x1° surface ship gravity and the 1x1° SEASAT altimeter derived gravity as well as the 1x1° surface ship gravimetric geoid and the 1x1° SEASAT altimeter derived geoid. The results show very good agreement in contour trends, wavelengths and amplitudes. Two types of residual geoids were computed. The first was obtained by subtracting the long wavelength features (degree 10 geoid of GEM10 standard earth model) from the 1x1° averaged SEASAT altimeter derived geoid. The second was obtained by further subtracting the geoid variations relating to the cooling of the plates. The Azores residual geoid high which centered over the Azores triple junction when only the long wavelength features were removed shifts its center east to the Azores Island after thermal effects were accounted for. This may indicate that the main axis of smaller scale (upper 600 km) convective upwellings does not lie beneath the present day spreading center but beneath the Azores Islands. Areas of extensive volcanism such as the Bermuda Rise and Cape Verde Rise in the older part of the ocean floor (ages greater than about 80 - 100 m.y.) exhibit relative residual geoid highs...