Continuous near-bottom gravity measurements made with a BGM-3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9 degrees 31 ' N and 9 degrees 50 ' N

A Bell BGM-3 gravimeter has been used to collect continuous, underway, near -bottom (3- to 10-m altitude) gravity measurements from the deep-diving sub mersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9 degrees 31'N and 9 degrees 50'N. Closely spaced (20- to 30-m) gravity me asurements were made along transects up to 8 km long in both regions. Repea tability of measurements made at the same location on different dives is si milar to 0.3 mGal. Along-track spatial resolution of anomalies is similar t o 130-160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest deter mined from the relationship between depth and free-water gravity anomalies varies greatly between 9 degrees 31'N and 9 degrees 50'N. Average upper cru stal densities of 2410 kg/m(3) for the 9 degrees 50'N area and 2690 kg/m3 f or the 9 degrees 31'N area were calculated. The different densities are not due to differing geometry of the Layer 2A-2B boundary or a regional cross- axis gravity gradient. Differences in porosity of the shallow crustal rocks , or a difference in the proportion of low density extrusives to higher-den sity dikes and sills within Layer 2A in these two areas, are the likely cau ses of the different upper crustal densities. Bouguer gravity anomalies nea r the EPR axis are primarily small amplitude (0.5-2 mGal), are a few hundre d meters across, and appear to be lineated parallel to the axis. Larger-amp litude Bouguer anomalies of up to 4 mGal were found at a few locations acro ss the crestal plateau and are associated with pillow ridges composed of la vas which are clearly younger than the surrounding seafloor. These ridges h ave distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5- t o 2-mGal anomalies observed on the summit plateau include areas of collapse d and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9 degrees 50'N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribu te to the observed Bouguer anomalies. The along-axis continuity of the grav ity anomalies is disrupted at a 75-m offset of the ASCT, suggesting that sh allow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the abilit y to conduct high-resolution, near-bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crust al structure of a fast spreading mid-ocean ridge develops and how it varies with the surface morphology.