ISOSTATIC COMPENSATION OF TECTONIC FEATURES OF THE MID-ATLANTIC RIDGE - 25-27-DEGREES-30'S

We investigate the isostatic compensation of tectonic features along the Mid-Atlantic Ridge 25-degrees-27-degrees-30'S through a detailed examination of the relationship between bathymetry and gravity anomalies. The study area includes three ridge segments, spreading at an average full rate of 35 mm/yr, their flanks out to about 6 m.y. old crust, and the intervening Rio Grande and Moore fracture zones. In a three-dimensional analysis of gravity and Sea Beam bathymetry data, we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models. Positive residual gravity anomalies over the northern wall of the Rio Grande fracture zone indicate that in spots the crust is 2-3 km thinner than average there, in contrast to the Moore fracture zone where little thinning is observed. The greater than average depths of the fracture zones as a whole are not locally compensated by thin crust, but the deepest basins within the active transform parts fo the fracture zones are partially compensated by thin crust or cooler mantle. A narrow linearidge that crosses one of the inactive branches of the Moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner, not thicker crust. There is no indication of thicker crust beneath topographic highs at the inside corners of ridge-transform intersections. Presumably, these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies. There is a residual gravity low associated with an unusually shallow 15- to 20-km section of the ridge segment between the Rio Grande and Moore offsets where the median valley virtually disappears. Our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust, much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation. Two-dimensional Fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented. This implies that the tectonic processes that control the gross ridge-transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure. Statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average, effective, elastic plate thickness in the study area is about 6 km.