Observers: Ken Rubin (transcript) and Bill Ryan (transcript)
Pilot: Bob Waters
Pilot: Robert Waters
Launch: 18° 14.57'S, 113° 21.63 W (x-y: 4300, 1000 m)
Summary: This dive investigated the EPR axis between 18°14.5' S and 18°15.5'S. During much of the dive we were shielded from the transponder navigation net by the topography of the axial graben, so a combination of dead reckoning and GPS fixes from the ship were used to construct much of the dive track. Eleven rock samples were taken of at least three lithologic units. Additionally, dive track 2 (from way points 2 to 3) followed a survey made by ABE on the previous night and confirmed bathymetric measurements made by the vehicle.
The dive began east of the axis in age 2.5 to 3 sheet flows and pillow lavas. After sampling at the landing site, we traversed the axial rift valley on a westerly track that took us over a series of faults and fissures on both walls of the graben. Faults were predominantly, but not exclusively, inward facing; many scarps had very fresh-appearing talus ramparts at their bases. Mid elevation walls on both sides of the valley where characterized by age 2 to 2.5 pillow and lobate lavas, transitioning to sheet flows of the same sediment cover age as nearer to the axis floor. The bottom of the axis on this traverse contained fissured pillow and lobate lavas of sediment cover age 1.5-2. Pockets of hydrothermal sediment and extinct high-temperature hydrothermal chimneys were observed in both the mid and upper elevations of the western wall of the rift valley. Just below the top of the west wall, the dive traversed an area of extensive collapse in age 2.5 lobate lavas, which was the site of both lava pillars and extinct high temperature hydrothermal constructs of up to ~8m in height. The summit of the western wall was age 3 sheet flows and lobate lavas.
The second two thirds of the dive consisted of three additional traverses of the axis. The two west to east traverses each reached sediment cover age 3 lavas near or at the top of the east wall of the valley, whereas the east to west traverse stopped at mid elevation on the western wall. Much of the geology on the upper and middle elevation reaches of the valley walls was the same as observed on the first dive traverse (from way point 1 to 2). However, on all 3 of the latter crossings, the valley floor was filled with significantly younger appearing lavas of sediment cover age 1.5. These lavas were primarily pillow constructs on track legs 2 and 3, whereas track leg 4 crossed pillows transitional to lobates. In each case, some fissuring was evident in the western reaches of these younger lava constructs. One lava was sampled from each of these crossings (samples 3355-4, -7 and -9) and were found to have distinctive mineralogy (olivine and plagioclase phenocrysts) and much less surficial alteration than those found on the valley floor on dive leg 1 (sample 3355-2) or on the valley walls (samples (3355-1, -3, -5, -6, -8, -10, -11).
On each of the dive tracks that traversed the young lava constructs, enhanced benthic faunal concentrations were observed in 10-20m wide bands on the age 1.5 lava flow. These were usually located near the center or eastern margin of the flow. In addition, enhanced biological activity was observed associated with an active diffuse flow venting location at the base of the western wall of the axial valley on dive track 4. This venting location was characterized by shimmering water issuing from a talus rampart at the base of a fault scarp. The talus was stained white and abundant anenomaes, crabs, and fish were populating an area of ~10m across.
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