Submarine volcanic features inferred from magnetic and gravity anomalies off Kume Island, mid-Okinawa Trough

SUMMARY The characterization of submarine volcanism associated with extension along continental margins is essential for understanding the evolution of rifting basins. In the Okinawa Trough, between the Eurasian continent and the Ryukyu Arc, submarine volcanism remains poorly understood. Here, we conducted detailed magnetic and gravity surveys aboard research vessels off Kume Island in the mid-Okinawa Trough to clarify the distribution of different types of submarine volcanic activities. Equivalent magnetization intensities were estimated from observed magnetic anomalies, and Bouguer gravity anomalies, calculated from the obtained free-air gravity anomalies, were used to estimate crustal thickness. Differences in crustal equivalent magnetization, Bouguer gravity anomaly and crustal thickness among bathymetric highs allowed us to identify four distinct groups of submarine volcanic edifices reflecting the evolution of rifting. Edifices of Group 1 are affected by basaltic magma intrusions associated with back-arc extension. Those of Group 2 have moderate equivalent magnetizations and Bouguer gravity anomalies compared to other edifices in the study area; the equivalent magnetization intensity in this group is comparable to the average intensity of the oceanic lithosphere. Edifices of Group 3 may have been affected by additional, voluminous magma supplies from active island arc magma sources. Finally, those of Group 4 are characterized by fewer basaltic intrusions compared to other groups and a lower degree of crustal thinning associated with back-arc extension. Moreover, the faults and fractures formed during the development of the Kerama Gap may have affected volcanism in our study area.