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Figure F1. Preliminary calculation of Mantle Bouger Anomaly (e.g., Lin et al., 1990) from shipboard gravity measurements in 1998 (Kelemen et al., 1998b; Matsumoto et al., 1998; Casey et al., 1998). Both figures use the same range of colors, representing slightly different values. A. Range is from about –35 (red) to +40 (pink) Mgal north of the 15°20'N FZ. B. Range is from about –60 (red) to +45 (pink) Mgal south of the 15°20'N FZ. These data suggest that the magma-starved region with abundant peridotite outcrops from 14°40' to 15°40'N lies on the periphery of large magmatic segments centered at ~14° and 16°N, with thick igneous crust in the segment centers.

Figure F2. Bathymetry and geology from 14° to 16°N along the Mid-Atlantic Ridge. Depth range is ~5400 (violet) to 1600 (red) m. Sample lithologies are compiled from all known dredging and submersible results. A. North of 15°20'N FZ. B. South of 15°20'N FZ. C. View from the north of the "megamullion" dive site, where a large low-angle normal fault is exposed on the seafloor. Open circles = mantle peridotite, solid circles = basalt.

Figure F3. Geochemical data on samples from the Mid-Atlantic Ridge. A. Low Na2O (upper panel data from the equator to 70°N, lower panel data from 10° to 20°N) in basalts. B. High Cr/(Cr + Al) in spinel (lower panel) and shallow axial depth (upper panel) can all be taken to indicate high degrees of partial melting. C. High La/Sm (upper panel data from the equator to 70°N, lower panel data from 10° to 20°N). D. High 206Pb/204Pb (upper panel) and high 87Sr/86Sr (lower panel). C and D are indicative of long-term enrichment of the mantle source in incompatible trace elements. All of these characteristics are observed along the Mid-Atlantic Ridge just south of the 15°20'N Fracture Zone (FZ). Basalt data compiled by Xia et al. (1991, 1992) and Casey et al. (1992). Spinel data and bathymetry from Bonatti et al. (1992) and Sobolev et al. (1991, 1992a).

Figure F4. A. Maps showing locations of conventional seismic refraction profiles (long white lines) and NOBEL experiments (numbered black lines) in 1997. B–D. Preliminary interpretation of data from the long refraction profile (R. Detrick and J.ÝCollins, pers. comm., 1998); (B) 2-D velocity model with contours labeled in kilometers per second, (C) indication of data coverage, which is sparse in the lower crust but sufficient to define large, lateral velocity variations (contours labeled in kilometers per second), and (D) traveltime data (circles) with model calculations (shading) for comparison. E. Comparison of two one-dimensional sections through the velocity model with a typical one-dimensional section for oceanic crust at the East Pacific Rise (EPR). MAR = Mid-Atlantic Ridge.

Figure F5. Proposed drill sites, including alternates. Please see Table T3 and site data sheets for submersible dives and lithologies associated with each site.

Figure F6. A. Schematic diagram drawn after Barnouin-Jha et al. (1997) showing results for the upper 50 km in a dynamic model of buoyancy-driven 3-D mantle flow beneath a slow-spreading ridge. Red = flow vectors in the horizontal plane, yellow = flow vectors in the vertical ridge-axis plane, blue = flow vectors in the vertical ridge-normal plane. This illustrates along-axis flow in the shallow mantle from segment centers to segment ends. Note spacing between upwelling centers is ~400 km and the region of melt generation is almost as long as the ridge segments. B. From Ceuleneer (1991), illustrating ductile flow vectors and shear sense inferred from peridotite fabrics in the mantle section of the Maqsad area, Oman ophiolite. Map area is ~17 km long x 14 km wide. Approximate location of inferred paleoridge axis is shown as a red line. C. Schematic diagram from Jousselin et al. (1998) showing their vision of mantle flow, based on observations from the Oman ophiolite, with a narrow zone of upwelling and a thin region of corner flow feeding a ridge segment that is three times longer than the diameter of the mantle upwelling zone. This model requires extensive subhorizontal ridge-parallel flow of residual mantle peridotite from the segment center to the segment ends. Although this geometry seems somewhat extreme and has not been produced in any 3-D dynamic model to date, it illustrates the type of highly focused solid upwelling that could produce the observed along-axis variation in crustal thickness on the Mid-Atlantic Ridge via 3-D focusing of mantle flow. Dynamic models such as that illustrated in A do not have sufficiently narrow zones of mantle upwelling and cannot reproduce the lengths of observed magmatic segments (~30–100 km). MOHO = Mohorovicic seismic discontinuity.

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