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

该项目涉及分析沿活跃的会聚板块边界的上板块的陆地地质，科科斯板块及其相关的海山和海脊被压在中美洲火山链之下。一般来说，会聚的板块边界可以用两个端元来描述：增长边缘和侵蚀边缘。在陆地和海洋地质学研究中，发生在沉积边缘(例如阿留申边缘、南开边缘、巴巴多斯边缘和俄勒冈-华盛顿边缘)的过程是众所周知的，但没有关于侵蚀边缘的类似知识(例如。日本、秘鲁、中美洲边际)。因此，主要研究人员正在制作新的基岩地质地图和横断面，并对哥斯达黎加太平洋海岸的海洋和河流阶地进行年代测定，以限制这种构造环境下的缩短、抬升速率和地表侵蚀。断层测绘和根据基岩地质学确定断层偏移量提供了过去200万年来缩短的数量的估计。对沿海海洋阶地的调查正在与放射性碳测年和光学激发发光测年相结合，以确定活动断层上的垂直运动速率和滑动速率。对河流阶地的分析和对火山岩和沉积岩中磷灰石颗粒的裂变径迹测年正在被用来确定侵蚀去顶的速度和总量。此外，在奥萨和尼科亚半岛，新发现的新近纪和第四纪海洋阶地和浅海沉积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该可以将这一记录追溯到新近纪晚期。最近在沿海发现了新的火山岩，可以用AR40/AR39方法测定其年龄，以更好地约束沿海变形带活动逆冲断裂的起始年龄。本研究中使用的方法旨在量化进入的海山和海脊对活动断裂作用和上板块地貌演变的影响。近海对这些水深特征的反应(构造侵蚀、下沉)和陆地反应(地壳增厚和抬升)之间存在明显的二分法。通过收集限制时间、运动学和变形速率的数据集，这项研究正在定义侵蚀性会聚板块边界的基本属性以及陆上和近海制度之间的关系。

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.