HR: 0800h
AN: V31C-0639    [Abstracts]
TI: Geophysical and Geological Data Document That Sediment Subduction and Subduction Erosion Recycle Large Quantities (Globally 2-3 km3/yr) of Continental Crust to the Mantle Wedge
AU: * Scholl, D W
EM: dscholl@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305 United States
AU: Hueme, R v
EM: rhuene@mindspring.com
AF: Department of Geology, University of California, Davis, CA, Davis, CA 95616, United States
AB: INTRODUCTION: At convergent margins the underthrusting ocean plate conveys continental material toward the mantle. Transport occurs in the subduction channel, a clastic layer separating the upper and lower plates. Charging of the subduction channel is effected by two kindred processes: (1) sediment subduction and (2) subduction erosion. Sediment subduction is the landward movement of ocean floor sediment beneath the margin's rock framework, and subduction erosion, a form of tectonic erosion, describes the transfer of upper plate material to the subduction channel. The principal evidence documenting sediment subduction is the small sediment prisms at most (70-80 percent) subduction zones. Subduction erosion is demonstrated by the near-trench occurrence of deeply subsided (4-5 km) nearshore deposits requiring crustal thinning on the order of 10-15 km. Other manifestations include a long record (more than 10-20 Myr) of inboard migration of the magmatic arc and the coastal exposure of arc rocks that were originally emplaced 50-150 km inboard of the shoreline area. RATES AND VOLUMES: For each lateral km of subduction zone, we estimate the average solid volume transport of crustal material toward the mantle is about 70 km3/Myr. Interestingly, this long-term volume rate is similar to the estimated rate of arc magmatism. Globally, the annual transport of subducted sediment and eroded forearc is 2-3 km3, of which 1/3 is subducted sediment and 2/3 is tectonically eroded debris. The thickness of the subduction channel moving material toward the mantle averages about 1.0 km. Because arc rocks are chiefly mantle melts and the global volume of exposed but once deeply underplated material is small, we presume that at most subduction zones at least 95 percent of subducted crustal matter is recycled to the mantle. COMPARISONS: Over a long period of geologic time the volume of recycled crustal material is impressively large. During the past 3 billion years the volume recycled through the mantle wedge is nearly equivalent to the Earth's existing volume of continental crust (9 trillion km3) and roughly 0.5 percent of that of the underlying mantle. The recycled volume is adequate to have imparted a distinct continental whiff to the Earth's mantle.
DE: 8400 VOLCANOLOGY
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005