Inner Forearc Deformation Along an Erosive Convergent Margin, Northeastern Japan

俯冲带在地球的长期物质平衡中起着重要的作用。地壳物质——它们既可以作为大陆边缘生长（增生边缘）的场所，也可以作为大陆边缘剥蚀（侵蚀边缘）的场所。本研究将量化日本东北部经典侵蚀辐合边缘前弧的缩短速率、侵蚀速率和地表抬升速率。将评估横跨前弧的物质平衡，并将永久应变积累模式与从地震和大地测量数据推断的弹性应变进行比较。侵蚀边缘在前弧的内部和外部经常表现出系统的相反行为：1)外前弧大部分是海底的，显示出延伸的证据，并有一个广泛的斜坡围裙，记录了边缘的沉降；2)具有沿海山脉、隆起和岩石的内前弧，显示出缩短和永久应变的证据。这种对偶在世界范围内沿侵蚀边缘的相对普遍存在表明一种潜在的遗传关系。该项目正在验证一种假设，即由基底侵蚀或逆冲作用引起的外前弧下沉与由底板或逆冲作用驱动的内前弧隆起相匹配。如果这一假设是正确的，那么基于外弧前沉降的全球大陆剥蚀的长期估计严重高估了大陆边缘物质的俯冲量。研究团队将采用多学科的方法，包括构造制图和断层相关的褶皱分析、地层年代学、低温热年代学（包括磷灰石裂变径迹分析和(U-Th)/He测年）、海洋阶地分析和地球动力学（热与变形）建模。在过去的二十年里，越来越明显的是，地球上的许多人，如果不是大多数人，都是如此。S俯冲带经历基底侵蚀，即外前弧通过板块边界向现有边缘楔的向上迁移而被移除。我们不知道的是，基底侵蚀速率持续了多久，被侵蚀的物质去了哪里？本研究通过检查主动系统的内部前弧来解决这些问题。查尔斯·达尔文在1846年的小猎犬号探险中首次注意到内前弧隆起，当时他在海平面以上一公里的海洋梯田上观察到海贝壳，并提出了南美洲西海岸地区隆起的观点。现代近海测深研究表明，达尔文所描述的局部隆起区域位于外弧侵蚀明显的区域的内侧。内前弧隆升可以平衡由于基底侵蚀引起的前弧沉降。

Subduction zones play an important role in the long-term mass balance of earth?s crustal materials - they can serve as either sites of continental margin growth (accretionary margins) or continental margin denudation (erosional margins). This study will quantify shortening rates, erosion rates, and surface uplift rates in the forearc of a classical erosive convergent margin in northeastern Japan. The mass balance across the forearc will be assessed and patterns of permanent strain accumulation will be compared with elastic strain inferred from seismic and geodetic data. Erosive margins often exhibit systematically opposed behavior in the inner and outer portions of the forearc, with 1) an outer forearc that is mostly submarine, showing evidence for extension, and having an extensive slope apron that records subsidence of the margin; and 2) an inner forearc with coastal mountains, uplift, and rocks that show evidence for shortening and permanent strain. The relative ubiquity of this couplet along erosive margins worldwide suggests a potential genetic relationship. This project is testing the hypothesis that outer forearc subsidence due to basal erosion, or underthrusting, is matched by inner forearc uplift driven by underplating, or overthrusting. If this hypothesis is correct, global long-term estimates of continental denudation that are based on outer forearc subsidence grossly overestimate the amount of continental margin material subducted. The research team will employ a multi-disciplinary approach that includes structural mapping and fault-related fold analysis, tephrachronology, low-temperature thermochronometry (including apatite fission track analysis and (U-Th)/He dating), analysis of marine terraces, and geodynamic (thermal and deformational) modeling.It has become increasingly apparent over the last two decades that many, if not the majority, of the earth?s subduction zones experience basal erosion, whereby the outer forearc is removed through upward migration of the plate boundary into the existing margin wedge. What is not known is how long are basal erosion rates sustained and where does eroded material go? This study addresses these problems by examining the inner forearc of an active system. Inner forearc uplift was first noted by Charles Darwin during the Beagle expedition of 1846 when he observed sea shells atop marine terraces located a kilometer above sea level and argued for regional uplift of the western coast of South America. Modern surveys of offshore bathymetry show that the local regions of uplift described by Darwin lie inboard of areas with pronounced outer forearc erosion. Inner forearc uplift may balance forearc subsidence due to basal erosion.