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

堪萨斯大学、华盛顿大学和罗格斯大学的研究人员合作解决了大陆边缘的俯冲作用是如何形成或侵蚀大陆的地质问题。更具体地说，它解决的问题是，有多少海洋沉积物被带到大陆边缘之下，又有多少沉积在中美洲近海“俯冲带”（尼加拉瓜和哥斯达黎加）的边缘上。这三名调查人员将对中美洲西海岸的火山进行实地采样，他们将分析这些岩石中的铍同位素10Be，从早期的工作中我们知道，从尼加拉瓜火山的高水平到附近哥斯达黎加火山的低水平，这些岩石中的铍同位素含量各不相同。高水平的10Be被假设为从海洋地壳上刮下来的年轻沉积物数量的替代指标，这些沉积物在地壳俯冲到中美洲海沟下面时被添加到大陆边缘。这里的研究将检查来自尼加拉瓜和哥斯达黎加的不同年龄的火山岩，它将包括尼加拉瓜-哥斯达黎加10Be过渡的时间变化的调查。该项目与1996年底由美国国家科学基金会支持的国际海洋钻探计划船JOIDES Resolution在哥斯达黎加近海进行的科学钻探计划密切相关。这次巡航将从大陆斜坡下收集沉积物样本，以检验沉积物俯冲和增生的直接证据。未来在尼加拉瓜近海进行类似研究的钻井段在JOIDES小组中排名很高，该小组制定了JOIDES Resolution钻井计划。尼加拉瓜-哥斯达黎加地区在这方面特别令人感兴趣，因为这里10Be信号的反差几乎与世界范围内观察到的一样大。由于10Be的高水平和低水平在地理上如此接近，它严格限制了可能涉及的沉积物增加之外的变量数量。因此，该项目有望提高我们对俯冲过程和大陆从洋壳俯冲形成的过程的认识。＊＊＊

9521716 Carr 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 which 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. ***