Inner Forearc Deformation Along an Erosive Convergent Margin, Northeastern Japan

俯冲带在地球--S地壳物质--的长期物质平衡中发挥着重要作用--它们既可以作为大陆边缘增长(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。这项研究将量化日本东北部典型侵蚀会聚边缘弧前的缩短速率、侵蚀速率和表面隆起速率。将评估弧前的物质平衡，并将永久应变累积模式与从地震和大地测量数据推断的弹性应变进行比较。侵蚀边缘通常在弧前的内外部分表现出系统的相反行为，1)外弧前，主要是海底的，显示出伸展的证据，并具有广泛的斜坡围裙，记录了边缘的下沉；2)内弧前，有沿海山脉、隆起和岩石，显示出缩短和永久应变的证据。这种对联在世界范围内的侵蚀边缘相对普遍，表明了一种潜在的遗传关系。该项目正在检验这样一种假设，即由于基底侵蚀或逆冲作用而导致的弧外下沉与由底侵或逆冲作用驱动的弧前内隆起相匹配。如果这一假设是正确的，那么基于外部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被俯冲的数量。研究小组将采用多学科方法，包括构造填图和与断层相关的褶皱分析、年代学、低温温度计时(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(热力和变形)模拟。在过去20年中，越来越明显的是，地球的许多(如果不是大部分)S俯冲带经历了基底侵蚀，因此通过板块边界向上迁移到现有的边缘楔形区，外部前弧区被移除。目前尚不清楚的是，基础侵蚀速率能持续多久，被侵蚀的物质会流向哪里？这项研究通过研究活动系统的内部前弧来解决这些问题。1846年，查尔斯·达尔文在Beagle探险队中首次注意到弧前内隆起，当时他在海拔一公里的海洋阶地上观察到了海壳，并主张南美洲西海岸的区域隆起。现代近海水深测量表明，达尔文所描述的局部隆起区位于弧前侵蚀明显的区域内。弧前内隆起可以平衡基底侵蚀引起的弧前下沉。

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.