HR: 10:20h
AN: V52A-01 INVITED    [Abstracts]
TI: Back-Arc Crustal Accretion: Interplay between Spreading Center Position and a Chemically Variable Mantle Wedge
AU: * Martinez, F
EM: fernando@hawaii.edu
AF: SOEST, University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822, United States
AU: Taylor, B
EM: taylorb@hawaii.edu
AF: SOEST, University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822, United States
AB: At distances greater than about 150 km from the arc, back-arc ridges are similar to their open-ocean counterparts. Closer to the arc the crustal thickness of back-arc ridges decreases then increases independently of spreading rate. Co-varying lava compositions indicate that these changes are controlled by mantle wedge chemistry. Two compositional variables that can affect crustal thickness are water content (increasing it) and chemical depletion (decreasing it). Both these variables generally increase toward the arc but observed crustal thickness variations suggest that depletion effects may extend farther from the arc before rapidly increasing water contents dominate. These inferences suggest that crustal thickness variations in back-arc basins reflect the position of the spreading axes relative to depletion and hydration gradients in the mantle wedge at the time the crust formed. Thus basin-wide geophysical mapping and tectonic reconstructions can reveal the patterns of these interactions in space and time. Two examples are the Mariana Trough and Lau Basin. At the Mariana Trough the spreading axes are closest to the arc at the northern and southern ends of the basin. Shallow crust there dominates the basin width suggesting that these spreading axes have remained close to the volcanic front during the entire opening history of the basin. The Lau basin has a complex western terrain with variable seafloor depths. This area has been interpreted as a rifted remnant arc but new seafloor mapping reveals morphologies similar to the actively spreading arc-proximal axes. This suggests that rather than tectonically rifted crust, the western terrain may reflect magmatic processes like those presently active on the arc-proximal axes, but which were abandoned when the these axes propagated southward. This model has implications for subduction material fluxes as arc-proximal spreading axes accrete thick arc-like crust with strong slab chemical signatures, but at rates that can be an order of magnitude or more than at the arc volcanic front itself. Thus the volume of this arc-like crust accreted during the opening history of a basin may represent a significant fraction of the total subduction output.
DE: 3001 Back-arc basin processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting