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Spreading rate dependence of gravity anomalies along oceanic transform faults

Abstract

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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Figure 1: Comparison of bathymetry and RMBA of the Siqueiros transform on the East Pacific Rise and Atlantis transform on the Mid-Atlantic Ridge, at the same map scale.
Figure 2: Compilation of ΔRMBA T - R values for the 19 transform systems analysed.
Figure 3: Results of 2D forward models showing the predicted ΔRMBAT - R.
Figure 4: Lateral variations in crustal thickness required to explain the observed RMBA in the Siqueiros transform system ( Fig. 1c ).
Figure 5: A spreading-rate-dependent model of crustal accretion and mantle upwelling based on observed RMBA calculations and morphological features at transform fault systems on slow- and fast-spreading ridges.

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Acknowledgements

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.

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Correspondence to Patricia M. Gregg.

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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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