Structure of volcanic centres at ultraslow spreading ridges revealed from local earthquake tomography

Melt distribution along ultraslow spreading ridges is characterized by strong focusing at widely spaced volcanic centers, rather than uniform axial accretion. This localized magmatism supports unusually frequent hydrothermal activity and high-temperature venting, posing a fundamental question: how is sufficient heat supplied and melt zones maintained within otherwise cold lithosphere? We present new local earthquake tomography results from two networks of eight ocean-bottom seismometer deployed around two confirmed hydrothermal vent fields on the Arctic Mid-Ocean Ridge—Aurora and Loki’s Castle. We inverted P- and S-phase arrival times of several thousand microearthquakes recorded over almost one year for P- and S-wave velocity structure and vp/vs ratio. Our tomography reveals heterogeneous lithospheric structures at both sites, with no clear evidence of large, sustained melt reservoirs. This contrasts with prominent low-velocity (vp/vs) anomalies at the Logachev and Segment 8 volcanic centers, which are indicative of extensive melt zones and are accompanied by seismic gaps, swarm activity, and circular magnetic anomalies.

The geophysical characteristics of Aurora and Loki’s Castle vent fields, located at ridge bends near regions of robust magmatism, differ significantly from those of Logachev and Segment 8. Despite the apparent absence of significant melt volumes, these sites exhibit long-lived hydrothermal activity. We propose that these differences may reflect distinct temporal stages in the life cycle of ultraslow spreading ridges or be related to the specific tectonic setting at ridge bends. Our findings highlight the complex interplay between magmatism, tectonics, and hydrothermal processes in ultraslow spreading environments.