GRAVITY-ANOMALIES, CRUSTAL THICKNESS, AND THE PATTERN OF MANTLE FLOW AT THE FAST SPREADING EAST PACIFIC RISE, 9-DEGREES-10-DEGREES-N - EVIDENCE FOR 3-DIMENSIONAL UPWELLING

Gravity, bathymetry, and crustal structure data from the 9 degrees to 10 degrees N segment of the fast spreading northern East Pacific Rise are used to investigate density variations in the mantle in order to d etermine the pattern of mantle upwelling and melt distribution beneath the segment. Mantle Bouguer gravity anomaly contours are nearly paral lel to the ridge axis, and there is only 3-4 mGal variation in axial m antle Bouguer anomalies along the entire segment. However, inclusion o f 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 degrees 50'N. Axi al subcrustal gravity anomalies increase by similar to 10 mGal between 9 degrees 50'N and 9 degrees 23'N, a gradient of 0.2 mGal/km. Since t he ridge axis has a uniform cross-sectional shape throughout the entir e segment with abundant evidence of magmatic activity along its entire length, it is unlikely that there are significant variations in crust al thermal structure other than at the ridge tips. We thus attribute t he subcrustal gravity anomalies to density variations within the mantl e. The distinct gravity low centered over the depth minimum provides q ualitative evidence that mantle upwelling is focused or enhanced at th at location. The effects of lithospheric cooling and subsidence away f rom the axis were removed from the bathymetry and free-air gravity ano malies to isolate residual gravity and gravity anomalies associated wi th the axis. The residual anomalies were modeled by considering the ax ial bathymetric high to be a flexural feature resulting from the upwar d buoyant load of a region of low-density material beneath the axis. A fter accounting for the isostatic effects of variations in crustal thi ckness, the remaining mass deficiency necessary to support the axial h igh and satisfy the gravity anomalies is not evenly distributed along the axis but is 35% greater beneath the axial depth minimum at 9 degre es 50'N than between 9 degrees 17' and 9 degrees 27'N. Mantle upwellin g in this segment thus appears focused or enhanced in the area of the depth minimum.