Seismic Structure of the Lithosphere-Asthenosphere System Beneath the Oldest Seafloor Revealed by Rayleigh-Wave Dispersion Analysis

Abstract

We analyze seismic records collected at the oldest (170–180 Ma) Pacific seafloor using broadband dispersion array analysis. Using ambient noise and teleseismic waveforms, we measure Rayleigh-wave phase velocities in a period range of 5–200 s that are inverted for array-average one-dimensional isotropic and azimuthally anisotropic shear-wave velocity depth profiles from the crust to a depth of 300 km. The high-velocity Lid and the low-velocity zone are well-resolved with velocity difference of ∼4%, whose transition occurs at depths between 80 and 100 km. The profile is compared with that obtained at the 130- and 140-Ma seafloor. Accounting for the cooling effect due to the plate age difference, the low-velocity zone of the oldest Pacific seafloor is ∼1.3% slower (∼110°C warmer) than that beneath the 140-Ma seafloor, suggesting the occurrence of some reheating process beneath the oldest lithosphere. The azimuthal anisotropy at shallow depths (<50 km) is significantly different between the western and eastern areas of the array where the peak-to-peak amplitudes are estimated to be ∼2.8% and ∼1.6%, respectively. The fast direction is nearly parallel to the past seafloor spreading direction (perpendicular to the magnetic lineation) in the west, while it largely deviates in the east. The observed difference in azimuthal anisotropy may represent complicated evolution dynamics of the infant Pacific plate that involved a ridge-ridge-ridge triple junction.