New observations on the distribution of past and present hydrothermal activity in the TAG area of the mid-Atlantic ridge (26 degrees 08 ' N)

Seafloor acoustic and photographic imagery combined with high-resolution ba thymetry are used to investigate the geologic and tectonic relations betwee n active and relict zones of hydrothermal venting in the TAG (Trans-Atlanti c Geotraverse) hydrothermal field at 26 degrees 08' N on the Mid-Atlantic R idge (MAR). The TAG field consists of a large, currently active, high-tempe rature mound, two relict zones (the Alvin and Mir zones), and an active low -temperature zone. The active mound and the Alvin relict zone lie along a s eries of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2 .5 km along these faults from the valley door onto the eastern wall, and co nsists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these moun ds is verified with near-bottom electronic still camera (ESC) images from t he Argo-II deep-towed vehicle, and is confirmed in at least one case with c ollected samples. Two of these mounds were previously unidentified. The exi stence of these mounds extends the length of the Alvin zone by similar to 0 .5 km to the south. Much of the Alvin relict zone appears to be buried by d ebris from a large mass wasting event on the eastern wall of the median val ley. The Mir zone, located on normal fault blocks of the eastern valley wal l, cannot be clearly identified in the sidescan data and no structural conn ections from it to the active mound or Alvin zone can be discerned. The act ive mound is located at the intersection of an older oblique fault set with the younger axis-parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major co ntrol on the location of hydrothermal activity by providing pathways for fl uid dow from a heat source at the ridge axis.