HR: 1340h
AN: B13A-0173    [Abstracts]
TI: Evidence for Pulsed Hydrothermal Venting from Young Abyssal Hills on the EPR Flank Suggests Frequent Seismic Pumping of Ridge Flank Fluid Flow
AU: * Haymon, R M
EM: haymon@geol.ucsb.edu
AF: UC Santa Barbara, Dept. of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Macdonald, K C
EM: macdonald@geol.ucsb.edu
AF: UC Santa Barbara, Dept. of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Benjamin, S B
EM: benjamin-sara@yahoo.com
AF: UC Santa Barbara, Dept. of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Ehrhardt, C J
EM: cehrhardt@umail.ucsb.edu
AF: UC Santa Barbara, Dept. of Geological Sciences, Santa Barbara, CA 93106 United States
AB: Although measured heat flow suggests that 40-50% of oceanic hydrothermal heat and fluid flux is from young (0.1-5 Ma) abyssal hill terrain on MOR flanks, hydrothermal vents in this setting rarely have been found. On the EPR flanks, seafloor evidence of venting from abyssal hills has been discovered recently at two sites: on $\sim$0.1 Ma seafloor at $10\deg$20'N, $103\deg$33.2'W ("Tevnia Site") and on $\sim$0.5 Ma seafloor at $9\deg$27'N, $104\deg$32.3'W ("Macrobes Site"). Manifestations of venting at these sites include: fault scarp hydrothermal mineralization and macrofauna; fault scarp flocculations containing hyperthermophilic microbes; and hilltop sediment mounds and craters possibly created by fluid "blow-outs." Hydrothermal deposits recovered at the $\sim$0.1 Ma "Tevnia Site" are fault breccias that record many episodes of brecciation followed by hydrothermal cementation (Benjamin et al., this session). Tubeworm casings, live crabs, and "dandelions" observed at this site indicate that the most recent episode of venting was active during, or shortly before, this site was visited with Alvin in 1994. To create the 200 m-high axis-facing fault scarp at Tevnia Site in 100,000 years, an average uplift rate of at least 2 cm/y is required. Since off-axis earthquakes located on abyssal hill fault scarps typically are $<$M5, it is likely that each episode of slip has small vertical displacement (a few cm). Therefore fault slippage must repeat on a decadal timescale to match the observed rate of hill uplift, probably breaking open the fault scarp and rejuvenating hydrothermal flow on a very frequent basis. In addition, close proximity to Clipperton Transform may subject Tevnia Site to frequent M5-M6 seismic events with strong ground shaking and hydraulic pressure pulses capable of breaking open subseafloor pathways clogged with fragile minerals. We hypothesize that the multiple brecciation/cementation events recorded in the Tevnia Site samples, and biological evidence for recent venting at the site, are evidence that hydrothermal plumbing systems are maintained semi-continuously over 100,000 years by tectonic shaking and reactivation every few years-to-decades as abyssal hills are uplifted on ridge flanks. Hyperthermophiles identified in microbial floc on the axis-facing fault scarp at the $\sim$0.5 Ma "Macrobes Site" are associated with chalcopyrite particles (Ehrhardt et al., this session). These observations indicate recent, transient venting of high-T (at least $250\deg$C), Cu-rich fluids from the fault scarp. The hilltop here has many meter-scale sediment mounds and craters (macs). These appear to be small mud volcanoes that have formed at different times from fluids expelled episodically through the thin sediments blanketing the hilltop. Formation of macs may be triggered by frequent (decadal) seismic events that also produce transient bursts of high T fluid flow from the adjacent fault scarp. Hyperthermophiles in the subsurface may flush out and bloom on the fault scarp during these events, and then remain dormant until the next event. The combined evidence from the two EPR ridge flank sites points to seismic pumping of EPR abyssal hill hydrothermal pulses every few years-to-decades. These frequent hydrothermal pulses may be important in sustaining ridge flank biota. Monitoring of EPR abyssal hill fault scarps is needed to test these ideas.
DE: 7220 Oceanic crust
DE: 8135 Hydrothermal systems (8424)
DE: 4804 Benthic processes/benthos
DE: 4832 Hydrothermal systems
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting