Evolution of the LAB in Old Oceanic Lithosphere Using Shear Wave Velocities Inverted from Rayleigh Wave Data Recorded By the Marine Seismic PLATE Project for 150 Ma Seafloor

Abstract:

A fundamental change occurs in seafloor subsidence in time evolved oceanic lithosphere older than 80 Ma which indicates a change from conductive cooling to more complex growth that is actively debated. There is a paucity of in situ seismic data, however, from seafloor older than the critical age threshold of 80 Ma that identify physical properties of the lithosphere or evolution of the LAB with age. Here, we invert for shear wave velocities using previous analysis of Rayleigh wave phase velocity data obtained from the PLATE project (Pacific Lithosphere Anisotropy and Thickness Experiment). We test a range of starting models, and apriori damping of model and data parameters The best fit to our phase velocity observations indicate shear wave velocities in the lithosphere are very high, extending to 4.75 km/s + 0.05 km/s at depths from the Moho down to 60 km + 25 km . A negative velocity gradient is observed from 70 km to 150 km depth with a minimum of 4.2 km/s + 0.3 km/s. The negative velocity gradient indicates the LAB occurs at 80 km + 25 km. The half-space cooling model predicts that detectable changes in velocity structure should extend to approximately 160 km for 150-160 Ma seafloor. Our OBS array is in an area of old seafloor unaffected by subsequent hot spot volcanism. We find lithospheric velocities that are significantly higher than previous global tomographic studies with much lower spatial resolution that inevitably include areas in which the seafloor has been affected by volcanic activity after formation. The anomalously low velocities in the LVZ indicates a compositional change of either partial melt or volatile content in the underlying asthenosphere.