GEOLOGY OF A SUBMARINE HYDROTHERMAL FIELD, MID-ATLANTIC RIDGE, 26 degree N LATITUDE.

A comparison of structural and lithologic features observed from two stereo-photographic traverses of the Trans-Atlantic Geotraverse (TAG) hydrothermal field, 26 degree N, Mid-Atlantic Ridge, with features of the French-American Mid-Ocean Undersea Study (Famous) hydrothermal site and the Mounds hydrothermal field of the Galapagos spreading center shows that all three areas have hydrothermal deposits with restricted and patchy lateral extent and are associated with closely spaced normal faults commonly of small displacement. No one lithology characterizes either the rocks or the sediments that underlie the hydrothermal deposits. Regions without hydrothermal manganese oxide deposits such as the rift valley at 37 degree N in the Famous area exhibit larger and fewer faults and lack the abundant permeable semiconsolidated breccia seen at 26 degree N. Thus structural factors which facilitate hydrothermal circulation by enhancing permeability appear critical to hydrothermal deposition. Structural featurs of the rift valley wall at 26 degree N include inward facing normal faulting, outward facing normal faulting, and outward tilting by rotation of fault blocks. These observations support a viscous drag mechanism of uplift of the rift valley walls.