An Abrupt Change in Ridge Axis Gravity with Spreading Rate

Small, C, Sandwell DT.  1989.  An Abrupt Change in Ridge Axis Gravity with Spreading Rate. Journal of Geophysical Research-Solid Earth and Planets. 94:17383-17392.

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The global mid-ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate. Fast spreading ridges have axial highs with little bathymetric relief and low-amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium. Slow spreading ridges have large axial valleys bounded by rugged topography (Macdonald, 1982) and large axial gravity troughs indicating that they are dynamically maintained. While this variation in ridge axis morphology with spreading rate has been observed, it has not been analyzed in a comprehensive manner. Moreover, it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate. Such observations would provide important constraints on models of ridge axis dynamics. Vertical deflection profiles collected by the Geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate. In this study, we have analyzed 44 Geosat profiles over ridges with spreading rates ranging from 14 to 155 mm/yr. In agreement with previous studies, we find that slow spreading ridges (<60 mm/yr) usually have high amplitude gravity troughs (40–100 μrad = 40–100 mGal), while fast spreading ridges (>70 mm/yr) are characterized by low-amplitude ridge axis highs (∼15 μrad). Unexpectedly, we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60–70 mm/yr. Ridge axis gravity signatures are highly variable for rates less than 65 mm/yr and very uniform at higher rates. The transition of the gravity signature appears to be more abrupt than the transition of the topographic signature, suggesting an abrupt change in the style of isostatic compensation with spreading rate. Published models of ridge axis dynamics do not explain this sharp transition.