Tectonic modification of axial crustal structure: Evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks

Bathymetry and gravity data of the northern Atlantic suggest that oceanic crustal structure on the flanks of the Mid-Atlantic Ridge is strongly modified by tectonic extension at spreading ridge segments. The seafloor along the zero-age isochron is systematically more elevated and the crust thicker near the segment midpoint than at the ends, suggesting focused magmatic accretion at segment centers. In contrast, seafloor older than 2-3 Ma is usually the shallowest at inside corners of ridge-offset intersections, where positive residual gravity anomalies indicate tectonically thinned crust. Coherence between residual gravity and bathymetry for zero-age crust is found at wavelength greater than similar to 20 km, which reflects the characteristic length of ridge segments, and is consistent with an effective elastic plate thickness of 1-3 km. No significant coherence, however, is observed on isochron profiles at ages of 2.5 Ma and older at all wavelengths. Flow-line profiles display coherence at wavelength greater than similar to 5 km, yielding an estimated effective elastic plate thickness of only 0.3-0.5 km. Synthetic fault models suggest that the flow-line coherence may be controlled by the relatively close spacing of fault-generated abyssal hills rather than reflecting the true elastic thickness of unfaulted lithosphere. Together these results indicate that the crustal structure of the zero-age crust is modified strongly by tectonic extension along the rift valley walls of the slow-spreading Mid-Atlantic Ridge. (C) 1998 Elsevier Science B.V.