A free-air gravity map of the Guiana Basin between 15° N. and 6° S. in the western equatorial Atlantic, using all available shipboard and pendulum data, is presented. The gravity field is interpreted in terms of short wave-length components directly related to topographic features and a long wave-length regional field which is independent of surface or basement relief. The regional field is negative throughout the survey area, varying from −15 to −40 mgal.

The magnetic anomalies over the large equatorial fracture zones indicate that the fracture zone trough is an area of zero or greatly reduced magnetization within a zone in which the magnetization is induced rather than remanent. Only about half of the gravity anomaly over the fracture zone can be assigned to topographic relief implying the presence of excess mass under the fracture zones. The gravity and magnetic evidence together suggest that large fracture zones serve as the site of intrusion of ultrabasic rocks from depth.

The deformation of the lithosphere due to the sediment load of the Amazon cone and the resulting gravity anomalies were computed for various flexural rigidities, using two-dimensional elastic beam theory. The value giving the best fit to the observed gravity anomalies in both wave length and amplitude is 2 × 1023 Newton meters (nt m) (2 × 1030 dyne cm). This implies an effective lithospheric thickness of 30 km. It is suggested that the lithosphere behaves somewhat as a Kelvin (viscoelastic solid) material in its response to imposed long-term loads, approaching a minimum apparent flexural rigidity of 2 × 1030 dyne cm asymptotically in a period of a few million years.

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