Crust–Mantle Architecture of the Pacific Plate Beneath the Japan Trench Petit-Spot Province Revealed by Seismic Imaging and Magnetic Anomalies

Petit-spot volcanoes are small-volume alkaline volcanic features that form on the seafloor of the outer-rise region of subduction zone. Their tectonic significance became evident in the early 2000s, when it was recognized that they are not related to mantle plumes or hotspots, but instead are likely associated with fracturing induced by plate flexure. The prevailing hypothesis suggests that as an oceanic plate approaches a subduction zone, it bends and undergoes extensional stress - particularly at its base - leading to the formation of deep-seated fractures. These fractures provide pathways for small volumes of melt to migrate upward and reach the seafloor. Petit-spot volcanoes also play an important role in modifying the physical and chemical parameters of the incoming plate. However, several aspects of their genesis remain uncertain, particularly how large-scale fractures develop within the lithosphere and how these structures manifest within the oceanic crust and uppermost mantle.

To gain insight into the geological architecture underlying the petit-spot volcanic province at the Japan Trench, we employ high-resolution seismic imaging techniques integrated with magnetic observations. Our analysis is based on a full-waveform inversion (FWI) velocity model constrained by ocean-bottom seismometer (OBS) data, complemented by reverse time migration (RTM), kinematic migration (KM), bathymetry, and magnetic anomaly data.

The FWI velocity model forms the foundation of other imaging results and is rigorously validated through real-synthetic data fitting, which demonstrate a strong agreement between both waveforms. Compared to previous tomographic models for this region, the improved kinematic accuracy and resolution enable imaging at greater depths and allow the interpretation of fine-scale features, highlighting velocity contrasts and structural interfaces. Migration results further confirm the robustness of the velocity model by accurately positioning deep structures near the Moho discontinuity and providing improved images of fault-related structures that cut through the crust and locally disrupt the Moho.

The velocity model reveals a layered oceanic crust with pronounced lateral heterogeneity. We identify three prominent low-velocity zones (LVZs) within the crust. The westernmost LVZ extends laterally beneath a strongly reflective Moho and is interpreted as a hydrated and mechanically weakened lower crust associated with bending-related faulting. Two additional, more localized LVZs are bounded by steeply dipping discontinuities that extend from the seafloor into the upper mantle, indicating deep fault zones capable of channeling fluids. Beneath the Moho, reduced seismic velocities in the uppermost mantle near the trench suggest significant serpentinization, consistent with the presence of bending-related faults and proximity to the subduction zone. Farther seaward, mantle velocities increase, indicating reduced hydration. A distinct high-velocity mantle domain is identified farther east, separated by sharp discontinuities that correlate with variations in Moho reflectivity and magnetic anomaly patterns.

Comparison with bathymetric and magnetic data reveals that deep seismic structures align with intersections of bending-related normal faults and abyssal-hill faults, as well as with a fossil propagating spreading center. These observations demonstrate that inherited tectonic fabric exerts a strong control on fault penetration depth, mantle hydration, and the crust–mantle architecture of the petit-spot volcanic province at the Japan Trench.