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

该项目包括分析沿活动辐合板块边界的上板块陆地地质，在那里，科科斯板块和相关的海山和山脊位于中美洲火山链的逆冲之下。一般来说，会聚板块边界可分为两个端元：增生边缘和侵蚀边缘。在陆地和海洋地质研究中，沿增生边缘（如阿留申、南开、巴巴多斯和俄勒冈-华盛顿边缘）发生的过程是众所周知的，但没有类似的侵蚀边缘（如南太平洋）的知识。日本、秘鲁、中美洲的边缘)。因此，主要研究人员正在绘制新的地图和基岩地质的横截面，并沿着哥斯达黎加太平洋海岸确定海洋和河流阶地的年代，以限制在这种构造环境下的缩短、隆起速率和表面侵蚀。从基岩地质中绘制断层图和确定断层偏移量提供了对过去200万年缩短量的估计。沿海海洋梯田的调查与放射性碳和光学激发发光测年相结合，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和沉积岩中磷灰石颗粒的裂变径迹测年被用来确定侵蚀剥落的速率和总量。此外，在奥沙半岛和尼科亚半岛新发现的新近纪和第四纪海相阶地和浅海沉积提供了前所未有的垂直构造活动记录。这些矿床的古地磁分析应将这一记录延伸至新近纪晚期。近年来，在沿海地区发现了新的火山岩，可以用Ar40/Ar39方法进行定年，从而更好地约束了沿海变形带活动逆冲断裂的起始年龄。本研究使用的方法旨在量化海山和海脊对活动断裂和上板块景观演化的影响。海上对这些水深特征的反应（构造侵蚀、下沉）和陆地对这些特征的反应（地壳增厚和隆升）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，本研究定义了侵蚀收敛板块边界的基本属性以及陆地和海上机制之间的关系。

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.