Isostasy of fracture zones in the Atlantic Ocean

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Abstract
The cross‐spectral method of studying the isostasy was applied to profiles of gravity and topography which run across fracture zones in the Atlantic Ocean between 40°N and 40°S; 30 sections in total were used, each section 400 km long. In wavelengths 25 to 80 km the coherence of the free‐air gravity anomaly and topography attains 0.8, while the phase is close to zero, which indicates that the disturbances in gravity are controlled by the topography and its isostatic compensation. The experimental isostatic response function (admittance) deduced in this study is compared with previous results which relied on different spatial distribution of data. A single mechanism of isostasy can explain well the gravity anomalies across fracture zones and across the median valley of the Mid‐Atlantic Ridge. A basic feature of this mechanism is the elastic lithosphere 6 to 9 km in thickness. We show that the topography and gravity anomalies over the Mid‐Atlantic Ridge creast are essentially three‐dimensional. Nevertheless, the admittance is reliably computed from a set of profiles by using the method of D. McKenzie and C. Bowin. This possibility is provided by the favorable pattern of the topography spectrum in the area under consideration.