Crustal Structure Across the Extinct Mid-Ocean Ridge in South China Sea From OBS Receiver Functions: Insights Into the Spreading Rate and Magma Supply Prior to the Ridge Cessation

The crust near an extinct mid-ocean ridge provides unique constraints on how its accretion and deformation responded to the cessation of spreading. Here we present crustal thickness and Vp/Vs measurements beneath 11 Ocean Bottom Seismograph sites that cross the South China Sea's extinct spreading axis. We find that the oceanic crust, which generally had only slight thickness changes once spreading started, abruptly thins at sites close to the extinct ridge axis. Abnormally high Vp/Vs ratios are obtained at several sites south of the ridge, indicating the presence of serpentine. These observations imply that, in its final stage, spreading changed to an ultraslow accretion style. As the axial crust thinned, normal faults and/or detachment faults began to form. Water could penetrate more deeply through these faults, and large fault slip could raise ultramafic peridotites to near or at the seafloor, creating favorable conditions for their enhanced serpentinization. Plain Language Summary Mid-ocean ridges can both form and die. As a ridge dies, what happens to melt generation beneath the ridge axis? Clues exist in the oceanic crust near an extinct ridge that was created during the ridge's final stages of seafloor spreading. We explore a well-known extinct spreading center within the South China Sea. Using ocean bottom seismometers and the receiver function technique, we determine the thickness of the magmatic crust, and a parameter (Vp/Vs) that reflects the structure of the underlying crust. We find that, approaching the extinct spreading axis, the crust, typically uniform in thickness, thins significantly. This indicates that the melt supply decreases relative to spreading rate during the final stages of spreading. The Vp/Vs ratio at three sites nearest to the extinct spreading axis is abnormally high. Our inference is that the spreading rate and magma production has decreased to the point where large crustal faults can develop at several locations along the ridge. These faults provide pathways for seawater to reach and react with the uppermost mantle, forming minerals with higher Vp/Vs ratios which are then uplifted to the seafloor as the faults grow.

Automated summary

The study of crustal thickness and Vp/Vs ratios near an extinct mid-ocean ridge in the South China Sea reveals that as spreading ceased, the oceanic crust abruptly thinned and shifted to an ultraslow accretion style. Abnormally high Vp/Vs ratios indicate the presence of serpentine, suggesting the formation of normal faults and the enhancement of serpentinization along the ridge axis.