Mid-ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge. Slow-spreading mid-ocean-ridge segments exhibit significant crustal thinning towards transform and non-transform offsets1,2,3,4,5,6,7,8,9,10,11,12, which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1,2,4,5,6,7,9,10,12. In contrast, fast-spreading mid-ocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis13,14,15. Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1,2,4,6, but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate- and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening. The most negative anomalies correspond to topographic highs flanking the transform faults, rather than to transform troughs (where deformation is probably focused and porosity and alteration are expected to be greatest), indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed. This finding in turn suggests that three-dimensional magma accretion may occur near intermediate- and fast-slipping transform faults.
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This work was supported by a National Science Foundation (NSF) Graduate Research Fellowship (P.M.G.), the NSF (M.D.B.), and the Woods Hole Oceanographic Institution Deep Ocean Exploration Institute (J.L. and L.G.J.M.). We are grateful for discussions with J. P. Canales, A. Cruse, H. J. B. Dick, D. Fornari, D. Forsyth, J. Georgen, J. Gregg, T. Grove, G. Hirth, D. Lizarralde, J. McGuire, M. Perfit, H. Schouten, D. Smith and the WHOI geophysics group. This manuscript was greatly improved by a review by R. Buck.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Gregg, P., Lin, J., Behn, M. et al. Spreading rate dependence of gravity anomalies along oceanic transform faults. Nature 448, 183–187 (2007). https://doi.org/10.1038/nature05962
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