Distribution of Fine-Scale Hydrothermal, Volcanic and Tectonic Features Along the EPR Crest, 17 15'-18 30'S: Results of Near-Bottom Acoustic and Optical Surveys

1997 American Geophysical Union Fall Meeting

R M Haymon, K C Macdonald, S Baron, L Crowder, J Hobson, P Sharfstein, S White, B Bezy, E. Birk, F Terra (All at: Dept. Geological Sciences, UCSB, Santa Barbara, CA 93106)

D Scheirer (Geological Sciences, Brown U., Providence, RI 02912)

D.J. Wright (Dept. of Geosciences, Oregon State University, Corvallis, OR 97331-5506)

L Magde (WHOI, Woods Hole, MA 02543)

C Van Dover (Inst. Marine Science, U. Alaska, Fairbanks, AK 99775)

S Sudarikov (VNIIOkeangeologia, St. Petersburg, 190121, Russia)

G Levai (Inst. Marine & Coastal Sci., Rutgers U., NJ 08901)

In Oct-Nov, 1996, the EPR axial zone was imaged with near-bottom ROV's to map the distribution of fine-scale features along a superfast ridge, and to determine the effects of along-strike variations in magma supply on hydrothermal processes. Using the DSL-120kHz sonar system, we collected sidelooking acoustic reflectance and phase bathymetry data within a 1 km swath continuous along the ridge crest from 17 15'S to 18 30'S (135 km). We then used the Argo II instrument package to conduct dense visual, acoustic, magnetic, and water column surveys within the DSL-120kHz sonar survey area.

The main Argo II survey ("Spike Survey") is continuous for 45 km along the EPR axis from 17 15'- 40'S. This survey is centered along-strike about a narrow intrusion (or "spike") that extends upward from the axial magma lens to < 800 mbsf at 17 24'-27'S (Detrick et al., 1993; Mutter et al., 1995). We identified 3 hydrothermal areas, one entirely north of the magma spike (17 15'-20'S); one directly above and just south of the spike (17 24'-29'S); and, a third, highly active area entirely south of the spike (17 32.5'-39'S). The distribution of vents appears to reflect 4th order ridge segmentation/magma supply. A small axial summit trough is present from 17 26'-29'S, above and just south of the spike. Extremely fresh lava flows are voluminous at the spike and outcrop southward all the way to the OSC at 17 42'S. A very high biomass of diverse organisms is present in the survey area: mobile fauna dominate areas of most recent lava flows, whereas sessile fauna are attached to sl! ightly older flows, suggesting that the youngest lavas erupted quite recently. Absence of extensive bacterial mats indicate that no eruptions were in progress in late 1996.

We conducted a second "N. Hump Survey" with Argo II from 18 23'-29'S, along a large axial summit graben. Smokers are active at the edge of a collapsed lava lake on the graben floor at 18 24.3'S. One Argo line was laid in along axis to visually image the transition between the morphologically distinct Spike and N. Hump areas.

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