The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global mid-ocean ridge system. Total spreading rates range from 12.7 mm/yr near Greenland to 6.0 mm/yr where the ridge disappears beneath the Laptev Shelf. Swath bathymetry and gravity data for an 850 km long section of the Gakkel Ridge from 5degreesE to 97degreesE were obtained from the U. S. Navy submarine USS Hawkbill. The ridge axis is very deep, generally 4700-5300 m, within a well-developed rift valley. The topography is primarily tectonic in origin, characterized by linear rift-parallel ridges and fault-bounded troughs with up to 2 km of relief. Evidence of extrusive volcanic activity is limited and confined to specific locations. East of 32degreesE, isolated discrete volcanoes are observed at 25-95 km intervals along the axis. Abundant small-scale volcanism characteristic of the Mid-Atlantic Ridge (MAR) is absent. It appears that the amount of melt generated is insufficient to maintain a continuous magmatic spreading axis. Instead, melt is erupted on the seafloor at a set of distinct locations where multiple eruptions have built up central volcanoes and covered adjacent areas with low relief lava flows. Between 5degreesE and 32degreesE, almost no volcanic activity is observed except near 19degreesE. The ridge axis shoals rapidly by 1500 m over a 30 km wide area at 19degreesE, which coincides with a highstanding axis-perpendicular bathymetric high. Bathymetry and side scan data show the presence of numerous small volcanic features and flow fronts in the axial valley on the upper portions of the 19degreesE along-axis high. Gravity data imply up to 3 km of crustal thickening under the 19degreesE axis-perpendicular ridge. The 19degreesE magmatic center may result from interaction of the ridge with a passively imbedded mantle inhomogeneity. Away from 19degreesE, the crust appears thin and patchy and may consist of basalt directly over peridotite. The ridge axis is continuous with no transform offsets. However, sections of the ridge have distinctly different linear trends. Changes in ridge trend at 32degreesE and 63degreesE are associated with a set of bathymetric features that are very similar to each other and to inside/outside corner complexes observed at the MAR including highstanding "inside corner'' ridges, which gravity data show to be of tectonic rather than magmatic origin.
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