HR: 0800h
AN: T31A-0473    [Abstracts]
TI: Opposing Trends in Crustal Thickness and Spreading Rate Along the Back-arc Eastern Lau Spreading Center: Controls on Ridge Morphology, Faulting, and Hydrothermal Activity
AU: * Martinez, F
EM: fernando@hawaii.edu
AF: School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Taylor, B
EM: taylorb@hawaii.edu
AF: School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Baker, E T
EM: edward.baker@noaa.gov
AF: NOAA/Pacific Marine Environmental Lab, 7600 Sand Point Way NE, Seattle, WA 98115 United States
AU: Resing, J A
EM: Joseph.Resing@noaa.gov
AF: University of Washington, JISAO-NOAA-PMEL, 7600 Sand Point Way NE, Seattle, WA 98115 United States
AU: Walker, S L
EM: sharon.l.walker@noaa.gov
AF: NOAA/Pacific Marine Environmental Lab, 7600 Sand Point Way NE, Seattle, WA 98115 United States
AB: At mid-ocean ridges (MORs), crustal production and spreading rate are tightly coupled primary variables that are thought to control major ridge features such as morphology, faulting, and hydrothermal activity. At back-arc ridges (BARs), in contrast, crustal thickness is observed to vary systematically with the position of the spreading center relative to the arc volcanic front, independent of spreading rate. This is thought to be due to hydration and depletion effects in the subduction mantle wedge which affect the magmatic productivity of the spreading center and is unlike at MORs where unusual crustal thickness variations are often attributed to mantle temperature anomalies. In the Lau back-arc basin the ~400 km long Eastern Lau Spreading Center (ELSC) approaches the arc volcanic front from north to south as the basin narrows. Correspondingly, ridge spreading rates decrease by more than half (97-39 mm/yr) as near-axis crustal thickness nearly doubles (~5.5-9 km). The opposing variations between spreading rate and crustal thickness at the ELSC thus allows observations, generally unavailable at MORs, of their decoupled effects on ridge morphology, volcanism, faulting, and hydrothermal activity. We investigate these effects as part of the first phase of RIDGE2000 Integrated Studies in the Lau back-arc basin. We used deep-towed side-scan sonar instruments (DSL120A and IMI30) to map the near-axis region within broader-coverage ship multibeam bathymetry and side-scan imagery swaths. An array of miniature autonomous plume recorders (MAPRs) attached to the deep-towed sonar's tow cables concurrently measured sea water optical backscatter in a vertical swath. Subsequent hydrocasts were made at identified plume sites to obtain water and particulate samples for chemical analysis. The data show that variations in ridge morphology are opposite to spreading rate trends at MORs, forming a peaked volcanic high at slow rates where the crust is thick and a deeper flat axis at fast spreading rates where the crust is thinner. Faults along the ELSC become larger and more widely spaced northward, also opposite to trends with spreading rate at MORs. Hydrothermal activity, as measured by plume incidence, increases with spreading rate, as at MORs, but reaches levels higher than the global MOR trend. The observations indicate that crustal thickness has a greater control on ridge morphology and faulting than spreading rate, even at fast rates where the thermal lithosphere should be thin. However, spreading rate, perhaps in combination with increased fault permeability, has a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge.
DE: 3001 Back-arc basin processes
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005