HR: 1340h
AN: T33D-0596    [Abstracts]
TI: Tectonic Windows Reveal Off-axis Volcanic and Hydrothermal Activity and Along-strike Variations in Eruption Effusion Rates
AU: * Macdonald, K C
EM: macdonald@geol.ucsb.edu
AF: Dept. Earth Science, UC Santa Barbara, Santa Barbara, CA 93106 United States
AB: Alvin transects of faulted escarpments 50-500m high provide tectonic windows to investigate the top 500m of oceanic crustal structure and lava stratigraphy. The Alvin archives were used to review dives from the East Pacific Rise, the Mid-Atlantic Ridge, the Juan de Fuca Ridge, the Blanco Trough, Cayman Trough and the Galapagos Spreading Center. A spreading rate dependence in lava morphology based solely on areal coverage(Bonatti and Harrison, 1988) was confirmed in scarp transects: mostly pillow lavas at slow spreading rates and sheet flows/lobate flows at faster spreading rates. More interestingly; there is a systematic variation within first, second and third order segments on intermediate and fast-spreading centers such that sheet/lobate flows dominate at segment centers and pillow flows and lava domes are more common at segment ends. This confirms earlier studies which were based on areal coverage (White et al, 2000, 2002, Soule et al 2005). This suggests higher eruption effusion rates and perhaps higher magma pressure and lower magma viscosity at segment centers relative to segment ends. This has important implications for the relationship between segmentation, magma supply, volcanism and hydrothermal activity (Haymon and White 2005). A conundrum remains; based on areal photographic surveys, why are pillow lavas so much more common off-axis than on-axis for intermediate to fast-spreading ridges? If there is an eruption cycle in which sheeted and lobate flows dominate early on, and pillow lavas dominate the waning stages of eruption (e.g. Ballard et al 1979), then more pillow lavas should be seen on axis than are seen on-axis in either areal or transect data. Another explanation is that pillow lavas off-axis are primarily produced by off-axis eruptions (except near segment ends, they may also occur as the pillowed terminations of channeled sheet and lobate flows; the association with channels will make this obvious.) Off-axis volcanism is also indicated by a steady increase in the number of isolated volcanic cones in the region 5-20 km off-axis (White et al, 1998, Alexander and Macdonald, 1996). Intact lobate and sheet flows are only observed in the top 5-10 m of the cross-sections. Below this, lobate and sheet flows are crushed to rubble by the overburden of later flows. When these rubble layers are correctly identified as crushed lobate/sheet flows, then the percentages of pillows/sheet flows/lobate flows seen in section are essentially the same as those reported in on-axis areal surveys, and are consistent with the along-strike variations documented by White et al. Whereas, the off-axis excess of pillow lavas is mostly explained by off-axis volcanism. In addition something unexpected was found: evidence for hydrothermal activity and associated microbial activity 26 km off-axis on the East Pacific Rise in the EPR R2K Integrated Studies Site area. This is the first documented off-axis hydrothermal field on a fast-spreading ridge that is not on a seamount (Haymon et al 2005). This activity is characterized by large areas (>100×100m) of mossy microbial floc containing hyperthermophiles associated with high-temperature metal-sulfide mineral particles. Visually similar floc was observed at a number of other locations off-axis on the Mid-Atlantic Ridge, Juan de Fuca Ridge and East Pacific Rise. This may provide opportunities for further exploration of off-axis hydrothermal activity in a variety of environments.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3036 Ocean drilling
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Tectonophysics [T]
MN: Fall Meeting 2005