RUI Collaborative Research: Crustal Thickening, Shortening, and Uplift in the Forearc of an Erosive Convergent Margin, Pacific Coast, Costa Rica

该项目涉及分析上板块沿着活跃的会聚板块边界的陆上地质，科科斯板块及相关的海山和海脊在中美洲火山链之下俯冲。 一般来说，会聚板块边界可以由两个端员：增生边缘和侵蚀边缘。 发生在沿着增生边缘的过程（例如，阿留申、南开、巴巴多斯和俄勒冈-华盛顿边缘）在陆地和海洋地质学研究中是众所周知的，但对于侵蚀边缘却没有类似的知识（例如，日本、秘鲁、中美洲边缘）。 因此，主要研究人员正在绘制新的基岩地质图和横截面图，并对哥斯达黎加太平洋沿岸沿着的海洋和河流阶地进行年代测定，以限制这种构造环境中的缩短、抬升速率和表面侵蚀。 断层图和基岩地质断层偏移量的确定提供了过去200万年缩短量的估计。 对沿着海洋阶地的调查正与放射性碳和光学受激发光测年相结合，以确定活动断层的垂直运动速率和滑动速率。 对河流阶地的分析和火山岩和沉积岩中磷灰石颗粒的裂变径迹年代测定正在被用来确定侵蚀去顶的速率和总量。 此外，在Osa和Nicoya半岛，新近发现的新近纪和第四纪海洋阶地和浅海沉积物提供了前所未有的垂直构造作用记录。 对这些沉积物的古地磁分析应将这一记录延伸到新近纪晚期。 近年来，在沿着发现了新的火山岩，可采用Ar 40/Ar 39方法测定其年龄，以更好地限定海岸变形带中活动逆冲断层的起始年龄。 本研究中使用的方法旨在量化传入海山和海脊对活动断层和上板块景观演变的影响。 近海对这些测深特征的反应（构造侵蚀、下沉）和陆上反应（地壳增厚和隆起）之间存在明显的二分法。 通过收集一个数据集，约束时间，运动学和变形率，这项研究是定义侵蚀收敛板块边界的基本属性和陆上和海上政权之间的关系。

This project involves analysis of the onland geology of the upper plate along an active convergent plate boundary, where the Cocos plate and associated seamounts and ridges are underthrust beneath the Central American volcanic chain. In general, convergent plate boundaries can be characterized by two end-members: accretionary margins and erosive margins. The processes that occur along accretionary margins (e.g., the Aleutian, Nankai, Barbados, and Oregon-Washington margins) are well known from studies of onland and marine geology, but there is no comparable knowledge for erosive margins (e.g.,. the Japan, Peru, Middle America margins). Consequently, the principal investigators are creating new maps and cross sections of bedrock geology and dating marine and fluvial terraces along the Pacific Coast of Costa Rica to place constraints on shortening, uplift rates, and surface erosion in such a tectonic setting. Mapping of faults and determination of fault offsets from bedrock geology provide an estimate of the amount of shortening over the last 2 million years. Surveys of marine terraces along the coast are being combined with radiocarbon and optical stimulated luminescence dating to determine rates of vertical motion and slip rates on active faults. Analysis of river terraces and fission track dating of apatite grains in both volcanics and sedimentary rocks are being used to establish the rate and total amount of erosional unroofing. Moreover, on the Osa and Nicoya Peninsulas, newly discovered deposits of Neogene and Quaternary marine terraces and shallow marine deposits provide an unprecedented record of vertical tectonism. Paleomagnetic analysis of these deposits should extend this record back into the late Neogene. Recently, new volcanic rocks have been discovered along the coast that can be dated by Ar40/Ar39 methods to better constrain the age of initiation of active thrust faulting in the coastal deformed belt. The methods that are used in this study are designed to quantify the effects of incoming seamounts and ridges on active faulting and landscape evolution of the upper plate. There is an apparent dichotomy between the offshore response to these bathymetric features (tectonic erosion, subsidence) and the onland response (crustal thickening and uplift). By collecting a dataset that constrains timing, kinematics and rates of deformation, this study is defining the fundamental attributes of erosive convergent plate boundaries and the relationship between onland and offshore regimes.