Shear Wave Velocity Structure and Evolution of Old Oceanic Lithosphere: Constraints from Rayleigh Wave Dispersion Across a Local Array of Ocean-Bottom Seismometers

Abstract:

The growth process of old ocean lithosphere has long been debated. Thermal models have included half-space conductive cooling, constant thickness cooling plate models with heat supplied to the base of the lithosphere by small-scale convection, and half-space cooling disrupted by impinging plumes. Seafloor subsidence and heat flux values for seafloor ages above 80 Ma are satisfied by the plate model, but previous Rayleigh wave studies have shown that thickening of the plate extends deeper than expected for those models. However, there is a lack of data from sufficiently old seafloor to describe end-member upper mantle structure in areas that have not been reheated by hotspot activity. We analyze Rayleigh wave data obtained from the PLATE project (Pacific Lithosphere Anisotropy and Thickness Experiment), where the seafloor is 150-160 Ma and has not been resurfaced by subsequent volcanism. Rayleigh wave phase velocities for periods from 18 to 125 s are inverted for shear wave velocities to study upper mantle structure. Shear velocities in the western region of our study area reach approximately 5.0 km/s in the top 20 km of the lithosphere and slightly higher in the eastern region. These velocities are higher than any reported in regional or global studies of the oceanic lithosphere that have poorer lateral resolution. These velocities are consistent with those predicted for conductive cooling of old, dehydrated oceanic lithosphere in previous petrologic studies. Shear wave velocities show a negative velocity gradient from 30 to 150 km depth, reaching a minimum velocity of ~4.3 km/s in the asthenospheric low velocity zone. Overall, the velocity structure for our study area beneath old oceanic seafloor is very similar to that predicted for a cooling half-space.