Along-Axis Gravity Gradients at Midocean Ridges - Implications for Mantle Flow and Axial Morphology

Along-axis gradients in mantle Bouguer anomalies (MBAs) were calculated for detailed gravity surveys at the Southwest, Central, and Southeast Indian ridges with half-spreading rates of 0.9, 2.3, and 3.1 cm/yr, respectively. MBA gradients at the very slow spreading Southwest Indian Ridge are 0.4-0.7 mgal/km, similar to the gradients found at the slow-spreading Mid-Atlantic Ridge, whereas gradients at the Central Indian and Southeast Indian ridges are 0.2-0.35 mgal/km. The results of this study, when combined with other published data, show that axial MBA gradients at ridge segments with an axial high are about 0.1 mgal/km, independent of spreading rate. Gradients at segments with a median valley are higher and generally decrease with increasing spreading rate. The spreading-rate-independent MBA gradient at ridge segments with an axial high represents the mantle gravity signal due to the focusing of upwelling within segments. The larger gravity gradients at segments with an axial valley result from the superposition of intra-segment variations in crustal thickness on this mantle signal. The consistent relation between along-axis MBA gradient and across-axis axial morphology suggests that the axial morphology is related to the efficiency of along-axis transport of melt.