Discovery of petrophysical and geophysical evidence for crustal formation processes during subduction initiation

1. Key Points

• By measuring the physical property of rocks drilled from the Izu-Bonin Arc, we revealed that rocks formed by volcanic activity immediately after subduction initiation differ markedly in crack density and elastic wave velocity from those produced during subsequent volcanic episodes.
• By relating seismic velocity structures beneath the seafloor to the rock physical properties, we demonstrate for the first time that an “early-formed crust” and a “later-formed crust” are actually superposed within the forearc crust.
• Linking the physical properties of rock samples with seismic velocity structure enables estimation of large-scale structure beneath the ocean floor and provides a pathway toward more detailed reconstruction of subduction-zone evolution using existing drilling cores and geophysical datasets.

2. Overview

The birth of subduction zones represents a critical turning point in the history of Earth’s plate tectonics. To understand the formation of continents and island arcs, it is essential to clarify how the crust was generated and evolved immediately after subduction initiation. However, direct evidence documenting the nature of volcanic activity responsible for such early crustal formation has been scarce, leaving this process poorly constrained for decades. In this study, we conducted detailed measurements of rock physical properties, including density, porosity, and P-wave velocity, using core samples recovered during International Ocean Discovery Program (IODP) Expedition 352 (Izu–Bonin forearc drilling). We demonstrate that rocks formed by volcanic activity immediately following subduction initiation exhibit P-wave velocities that differ markedly from those of rocks produced during later volcanic stages, primarily due to differences in internal crack density. By integrating these results with seismic velocity structures observed around the drilling sites, we show that the crust was constructed through two distinct stages: volcanic activity accompanied by seafloor spreading shortly after subduction initiation, followed by volcanism after spreading had ceased. This study provides the first geophysical demonstration of how forearc crust is constructed beneath nacent subduction zones.

These results have been published in Communications Earth & Environment on April 9th (Japan time). The research was conducted with the support of a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (KAKENHI; JP22H01337), the cooperative research program of the Center for Advanced Marine Core Research, Kochi University (Grants 16A047, 16B041, 17A058, and 17B058), and a “Strategic Research Projects”grant from the Research Organization of Information and Systems.