Inner Forearc Deformation Along an Erosive Convergent Margin, Northeastern Japan

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.