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

Abstract

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 ∼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 ∼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.