Jiabin Zhao

The mechanisms of complex Moho reflections at fast‐ and intermediate‐spreading ridges remain unclear. A potential key factor is hydrothermal activity, which at mid‐ocean ridges may result in serpentinization of peridotites under the stability field of antigorite (400–600°C) or chrysotile‐lizardite (<400°C). Here, we calibrated the relationship between serpentinization degree, density, and seismic velocities of peridotites at 200 MPa. Based on physical properties of rocks in the oceanic lithosphere, we established seismic velocity and density models of a sharp or layered crust‐mantle boundary with different serpentinization types. Synthetic seismograms demonstrate that the oceanic Moho reflections are controlled by both structure and serpentinization of the crust‐mantle boundary. The impulsive Moho may correspond to a sharp mafic‐ultramafic boundary (petrological Moho) with <10% serpentinization, or the bottom of 60%–80% antigorite peridotites. The Moho reflection becomes invisible when antigorite percentage decreases from 60%–80% (top) to 0% (bottom) in the uppermost mantle for a sharp mafic‐ultramafic boundary. Besides the layered crust‐mantle boundary, the diffusive Moho could also result from the sharp mafic‐ultramafic boundary with 10%–60% or 80%–100% antigorite peridotites, or chrysotile‐lizardite peridotites overlying antigorite peridotites. Using synthetic seismograms, the diffusive Moho at 6–9 km to the west of the East Pacific Rise 9°44′N can be interpreted by 30% antigorite peridotites, suggesting localized hydrothermal circulation close to the fast‐spreading mid‐ocean ridges. Moho reflections and wide‐angle PmP waveforms at mid‐ocean ridges can provide new clues to decipher the interplay between magmatic, tectonic, and hydrothermal processes in the oceanic lithosphere.

The crustal structures of the continent-ocean transition (COT) zone and oceanic domain are key to revealing the tectono-magmatic evolution from rift to drift and the following seafloor spreading. We develop a comprehensive study of the deep seismic reflection imaging and tomographic inversion of a wide-angle seismic line that runs across the COT and extinct spreading center of the Southwest Subbasin (SWSB) in the South China Sea. We reveal a low-velocity (<3 km/s) region in the shallow upper crust of the Longmen Seamount, which may represent serpentinized mud or volcanoclastic. The mature oceanic crust is approximately 4–6 km thick with high-velocity bodies (7.2–7.5 km/s) overlapping the Moho, reflecting relatively rich magmatic additions during seafloor spreading. The northern and southern COT segments exhibit a prominent long-wavelength high magnetic anomaly and synbreakup volcano, indicating magmatic additions in the lower crust and lava flows with magma ascending along faults to the surfaces. However, the COT in the south is wider than that in the north. In addition, the southern COT is characterized by limited PmP reflections, well-developed rotated fault blocks and ocean-ward synrifting faults, and a low-velocity (7.5–7.8 km/s) upper mantle, suggesting that the southern COT is probably underlain by a local serpentinized mantle during continental lithosphere breakup. The differences between the southern and northern COTs on the origin of high-velocity lower crustal layers and faulting styles imply asymmetric continental breakup processes in the SWSB.