HR: 10:20h
AN: T42A-01    [Abstracts]
TI: On The Significance Of Tectonic Underplating Beneath Continental Margins
AU: * Ducea, M
EM: ducea@email.arizona.edu
AF: U Arizona, 1040 E Fourth Street, Tucson, AZ 85721, United States
AB: More than half of the modern subduction margins are eroding - that is the preserved amount of terrigenous and pelagic sediment accreting to the trench is less than predicted for the lifetime of the trench. In many cases in fact, long-lived trenches are entirely sediment starved. Lateral continental disassembly and/or subduction erosion (sediment subduction and tectonic underplating) are the possible suspects at eroding margins. Tectonic underplating is the process of burial of accretionary wedge sediments and forearc sequences by transferring them from the upper plate to the lower plate at the subduction interface followed by reattachment of these materials to the deep crust of the upper plate. I will give examples for both and establish some general rules for differentiating among the two in the geologic record. As a specific and more detailed example, I will then use the Acapulco trench of southern Mexico to demonstrate that at least about 90% of the sediment mass predicted for this trench wedge is missing and was most likely removed by subduction erosion. This calculation integrates marine seismic, drilling and forearc thermochronology for the past 25 M.y. of the life of the Acapulco trench. Most conceptual models that take sediment recycling into consideration assume that the sediment is almost entirely recycled into the mantle, with perhaps the exception of a small fraction of material returned to the upper plate via arc magmatism. In contrast, I suggest that tectonic underplating is a major process at many convergent margins. The modern lithospheric architecture of southern Mexico from the Acapulco trench to the Mexican Volcanic Belt is a world-class example of tectonic underplating, and I will present evidence for that. Tectonic underplating is fundamentally a mechanism that: (1) provides a way to deliver upper crustal materials to the lower crust in arc settings, and significantly weakens the upper plate, (2) temporarily localizes some subduction megathrusts entirely within the crust, (3) leads to a hotter-than-normal subduction system that can influence the crustal melting function in arc regions.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8104 Continental margins: convergent
DE: 8158 Plate motions: present and recent (3040)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly