A Structural Analysis of Sediment Subduction Off Nicaragua: Why the 10Be Anomaly?

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9523409 Morris This collaboration among researchers at University of Kansas, Washington University and Rutgers University addresses the geological question of how subduction at a continental margin builds or erodes the continent. More specifically, it addresses the question of how much ocean sediment is carried down beneath the continental margin compared to how much is plastered against the margin at the "subduction zone" offshore of Central America (Nicaragua and Costa Rica). The three investigators will carry out a field program sampling volcanoes along the west coast of Central America, and they will analyze these rocks for 10Be, an isotope of beryllium which is known from earlier work to vary from high levels in Nicaragua volcanoes to low levels in volcanoes from nearby Costa Rica. The high levels of 10Be are hypothesized to be a proxy indicator of the amount of young sediment which is scraped off of the oceanic crust and added to the continental margin as crust is subducted beneath the Middle America Trench. The study here will examine volcanic rocks of various ages from Nicaragua and Costa Rica, and it will include investigation of the time variability of the Nicaragua-Costa Rica 10Be transition. The project ties closely to planned scientific drilling by the NSF-supported, international Ocean Drilling Program ship JOIDES Resolution offshore Costa Rica in late 1996. That cruise will collect samples of sediments from beneath the continental slope to examine the direct evidence for subduction and accretion of sediments. A future drilling leg offshore Nicaragua to perform a similar study there is highly ranked by the JOIDES panels which set the JOIDES Resolution drilling program. The Nicaragua-Costa Rica region is of particular interest in this regard because the contrast in 10Be signal here is nearly as large as that observed worldwide. Because the high and low 10Be levels are so close geographically, it tightly constrains the number of variables aside from sediment accretion whic h may be involved. The project, therefore, can be expected to improve our understanding of the subduction process and the processes of continental growth from ocean crust subduction. ***