1996 Articles

Gravity anomalies, crustal thickness, and the pattern of mantle flow at the fast spreading East Pacific Rise, 9°-l0°N: Evidence for three-dimensional upwelling

Wang, Xuejin; Cochran, James R.; Barth, Ginger

Gravity, bathymetry, and crustal structure data from the 9° to 10°N segment of the fast spreading northern East Pacific Rise are used to investigate density variations in the mantle in order to determine the pattern of mantle upwelling and melt distribution beneath the segment. Mantle Bouguer gravity anomaly contours are nearly parallel to the ridge axis, and there is only 3- 4 mGal variation in axial mantle Bouguer anomalies along the entire segment. However, inclusion of the gravity effects of variations in crustal thickness and structure results in a significant change in the pattern of gravity anomalies. The "subcrustal" gravity anomalies show a very distinct gravity low centered over the segment axial depth minimum near 9°50'N. Axial subcrustal gravity anomalies increase by ~ 10 mGal between 9°50'N and 9°23'N, a gradient of 0.2 mGal/km. Since the ridge axis has a uniform cross-sectional shape throughout the entire segment with abundant evidence of magmatic activity along its entire length, it is unlikely that there are significant variations in crustal thermal structure other than at the ridge tips. We thus attribute the subcrustal gravity anomalies to density variations within the mantle. The distinct gravity low centered over the depth minimum provides qualitative evidence that mantle upwelling is focused or enhanced at that location. The effects of lithospheric cooling and subsidence away from the axis were removed from the bathymetry and free-air gravity anomalies to isolate residual gravity and gravity anomalies associated with the axis. The residual anomalies were modeled by considering the axial bathymetric high to be a flexural feature resulting from the upward buoyant load of a region of low-density material beneath the axis. After accounting for the isostatic effects of variations in crustal thickness, the remaining mass deficiency necessary to support the axial high and satisfy the gravity anomalies is not evenly distributed along the axis but is 35% greater beneath the axial depth minimum at 9°50'N than between 9° 17' and 9°27'N. Mantle upwelling in this segment thus appears focused or enhanced in the area of the depth minimum.