Gravity anomalies, crustal thickness, and the pattern of mantle flow at the fast spreading East Pacific Rise, 9°–10°N: Evidence for three‐dimensional upwelling

Abstract

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.

Mantle upwelling, magmatic differentiation, and the meaning of axial depth at fast-spreading ridges

“…Such mantle is assumed to be hotter, to contain more melt, and to supply more magma to the ridge crest in comparison with mantle beneath regions where the rise axis is deeper. Previous studies of topography and gravity assume that the excess supply of magma beneath axial highs is redistributed in the rise-parallel direction, an issue we address further below, and that intrasegment highs are underlain by segment-scale mantle diapirs (Barth and Mutter, 1996;Macdonald, 1998;Macdonald et al, 1988;Wang et al, 1996). The residual mantle Bouguer anomaly (RMBA, the free air gravity anomaly corrected for the topography of both the seafl oor and the seismically measured Moho, and for the mantle thermal structure due to the ridge geometry; see the GSA Data Repository 1 ) for the East Pacifi c Rise between the Siqueiros and Clipperton transforms on June 9, 2015 geology.gsapubs.org Downloaded from argues against this interpretation.…”

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…Carbotte et al 2000;Donnelly 2002). Some studies suggest the presence of a melt upwelling centre at c. 9850 ′ N, with alongaxis melt flow away from this region (Barth & Mutter 1996;Wang et al 1996), although other interpretations have also been advanced (Toomey & Hooft 2008).…”

Along‐strike variation of the sheeted dike complex in the Oman Ophiolite: Insights into subaxial ridge segment structures and the magma plumbing system

“…Segment centers, usually shallowest and largest in cross-sectional area, are proposed to be the loci of magma upwelling, from which melt is redistributed toward the distal ends of the segments [e.g., Batiza, 1996;Macdonald, 1998;Wang et al, 1996]. However, seismic studies on an overlapping spreading center (OSC) at 9°N on the East Pacific Rise (EPR) suggest existence of thicker crust and extrusive strata than the segment center [Barth and Mutter, 1996;Harding et al, 1993;Kent et al, 2000].…”

“…[2] The segmentation of mid-ocean ridges coincides with along-axis variations in lava geochemistry, gravity anomalies and seismic velocity structures [e.g., Batiza, 1996;Barth and Mutter, 1996;Langmuir et al, 1986;Macdonald, 1998;Wang et al, 1996]. Segment centers, usually shallowest and largest in cross-sectional area, are proposed to be the loci of magma upwelling, from which melt is redistributed toward the distal ends of the segments [e.g., Batiza, 1996;Macdonald, 1998;Wang et al, 1996].…”