Tomographic Constraints on a Mid‐Crustal High‐Velocity Body Beneath West‐Central Taiwan: Implications for Passive‐Margin Mafic Additions

Abstract Using dense seismic data sets, we present a new 3D velocity model of Taiwan that images a prominent mid‐crustal (∼20–30 km) high‐velocity body beneath west‐central Taiwan. The inclusion of high‐quality post‐2012 recordings from the Central Weather Administration Seismic Network (CWASN) ensures the elimination of uncorrectable timing errors. In addition, a machine learning–based phase picker was applied to the entire data set to improve the consistency and accuracy of phase arrival identification. The resulting model generally aligns with previous tomographic studies. Structures inferred from velocity gradients in the tomographic profiles largely correspond to mapped faults and geological unit boundaries in Taiwan. The model also images a pronounced mid‐crustal (∼20–30 km) high‐velocity anomaly beneath west‐central Taiwan. Under representative P–T conditions, Vp–Vs–density comparisons indicate the best match with mafic compositions, though the interpretation is not unique. This mafic interpretation is compatible with passive‐margin mafic additions (underplating and/or intrusions). The anomaly coincides with reduced seismicity below ∼20 km and depth‐dependent stress orientations, consistent with a relatively competent mid‐crustal volume. Geological and geophysical similarities with the Dongsha Rise further suggest a possible shared tectonic and magmatic origin, likely linked to mafic underplating during South China Sea rifting. These findings improve our understanding of structural highs along passive continental margins and their role in influencing crustal deformation in the Taiwan orogen. The new model also provides a robust framework for future waveform‐based seismic imaging.