Revisiting seafloor magnetotelluric data and imaging local high-resolution electrical conductivity structure in the western Pacific

The electrical structure of oceanic lithosphere and underlying asthenosphere provides crucial constraints on upper mantle dynamics, complementing seismic observations to distinguish between competing mechanisms controlling mantle properties. We present an enhanced magnetotelluric (MT) analysis of the Philippine Sea, incorporating up to three years of continuous ocean bottom electro-magnetometer (OBEM) data that expand upon previous one-year analyses. Our study employs both standard and generalized remote reference processing methods using land-based geomagnetic stations, yielding significantly reduced error bars across the entire period band (640 to 245,760 s) and increased coherence between observed and predicted electric fields. Conductivity inversions were performed for each site, incorporating 3D topographical heterogeneity overlaying a 1D mantle model to account for topographic effects in MT responses. We observed for the first time that the induction vector calculated using the final 1D model with 3D topography closely matched the induction vectors derived from observed magnetic data. Since the induction vectors were not incorporated in the inversion analysis, this independent validation demonstrates that non-1D features of both MT and induction vectors are effectively explained by topography effects, underscoring the critical importance of topographic corrections in marine electromagnetic studies. Our 1D conductivity models revealed a pronounced high-conductivity anomaly at depths of 30 to 125 km beneath sites T01, T02, and T04 in the western West Philippine Basin (WPB), with peak conductivity values reaching 0.1–1.0 S/m. Thermal modeling using experimental hydrous olivine conductivity relationships indicates this enhancement requires either elevated temperatures, enhanced water content of 0 to 0.02 wt%, or/and small degrees of partial melting. This finding aligns with previous seismic observations of slow-velocity anomalies in the same region. The spatial correlation of high-conductivity sites with the extinct WPB spreading center suggests the anomaly reflects volatile enrichment from plume–ridge interaction basin formation. These results demonstrate that extinct spreading systems associated with mantle plumes may maintain conditions conducive to partial melting over geological timescales and provide new insights into the physical properties of the uppermost mantle beneath the western Pacific, helping constrain the extent and depth of partial melting in complex convergent margin settings.