HR: 10:45h
AN: T32B-02 INVITED    [Abstracts]
TI: Thoughts on the Significance of the East Pacific Rise "Undershoot" Seismic Results
AU: * Macdonald, K
EM: macdonald@geol.ucsb.edu
AF: Dept Earth Science, UCSB, Santa Barbara, CA 93106, United States
AU: Fox, P J
EM: fox@iodp.tamu.edu
AF: IODP, Texas A&M U., College Station, TX 77845, United States
AU: Haymon, R
EM: haymon@geol.ucsb.edu
AF: Dept Earth Science, UCSB, Santa Barbara, CA 93106, United States
AB: Reported EPR seismic results between the Clipperton and Siqueiros transforms indicate that low velocity zones in the uppermost mantle (MLVZ's) correlate well with 3rd order tectonic/volcanic segments (Toomey et al., 2007). This adds to other geologic observations which correlate with segmentation: axial depth, cross-sectional area, crustal magnetization, inferred eruption temperature and fractionation (based on MgO), patterns of hydrothermal activity, inferred eruption effusion rates (based on lava morphology, ie, sheets/lobates vs pillows), and average lava age (based on sediment cover). These data are all consistent with the model that, in a time averaged sense, segment middles mark the surface expression of hotter, more buoyant upper mantle. Stacking several segments together to minimize effects of temporal variations at any one location, the data support the model that magma supply is enhanced near segment centers and is reduced or more episodic near segment ends. One of the 7 MLVZ's occurs up to 10 km off-axis near 9 20-9 35N; 6 occur beneath the EPR axial high. Prior segmentation models did not consider off-axis mantle upwelling effects on axial volcanic processes, due to lack of data, but it is unsurprising if this occasionally occurs. It may explain decreased hydrothermal activity along this one segment where the apparent reduction in axial melt supply could stem from lateral melt transport from off-axis. The new ridge segmentation model proposed by Toomey et al. (2007) is inspired by measurements showing seismic anisotropy canted 5-8 deg. ccw from the EPR topographic axis, which led Toomey et al. to "conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise". To produce this skewness, the asthenosphere must somehow lead the lithosphere in responding to a change in spreading direction, and this must occur virtually everywhere to explain the ubiquity of ridge segmentation on all spreading ridges. Assuming skewness is adequately resolved in their data, we offer a much simpler local explanation for it. Magnetic anomalies show that the EPR axis has rotated ccw by an amount similar to the observed skewness in response to a recent change in spreading direction. The long 1st order segments N of Clipperton and S of Siqueiros have rotated in response to this change, while the relatively short segment between these transform faults has not. Why? Since Siqueiros steps right and Clipperton steps left, there is transpression across Clipperton and opening across Siquieros. Thus rotation of the segment trapped between these transforms is impeded, while the asthenosphere below is free to rotate in response to the change in spreading direction. Also, this is the shortest 1st order segment bounded by large transforms along the Cocos-Pacific boundary, which may further impede adjustment (interestingly, 3rd and 4th order segments within the axial high ARE starting to show ccw adjustment). This explains skewness in the study area without appealing to a more astonishing universal skewness of mantle upwelling relative to the spreading axis as the cause of ridge segmentation; and, it allows ridge segmentation to exist where spreading directions have not changed. Neither the MLVZ ~10 km off-axis nor a mantle high conductivity zone further off-axis (Constable et al.) produce corresponding surficial evidence (e.g., off-axis volcanism, significant shallow depth anomaly, and/or bright backscatter images). Possibly the MLVZ has only recently arrived in this location. It also is possible that the MLVZ is a geologically ephemeral expression of asthenospheric flow representing instabilities in upwelling.
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting