Masters Thesis

Rayleigh wave phase velocities and anisotropy of old oceanic lithosphere in the Pacific

The processes that control the growth and the formation of the lithosphere are crucial for our understanding of plate tectonic theory. Simple conductive cooling of lithospheric material accurately describes seafloor behavior such as seafloor subsidence and heat flow for young seafloor, but does not predict the behavior of the lithosphere at ages greater than 80 My. More complex models of lithospheric growth and mantle flow have been invoked to explain this departure at old ages such as small scale convection upwelling plumes, viscous fingering, crustal thickness variations, or return flow opposing plate motion. Global seismic tomography studies indicate seismic anisotropy is weaker beneath old seafloor compared to younger regions, but are often limited to land stations, which record oceanic raypaths and lack resolution to measure oceanic lithospheric thickness and flow structure predicted by the multitude of models. Here I present Rayleigh wave tomography results from the deployment of 16 ocean bottom seismometers (OBS)in the NW Pacific on 150-160 My seafloor. This location is ideal because it displays a magnetic bight which provides unique opportunity to sample two regions with the same age but different fossil spreading directions. Rayleigh wave data are analyzed for 86 earthquake events at periods from 18 to 144 seconds. A rigorous set of corrections were applied to OBS data to maximize the data set utilizing differential pressure gauges, and accounting for tilt and infragravity water waves to increase the period range and data recovery rate beyond previous marine studies. Inversions for phase velocity averaged over the study area indicate that Rayleigh wave velocities are 1.3 % higher than previous studies for seafloor with ages 110+ My, but the overall shape of the dispersion curve at long periods remains similar suggesting that the lithosphere continuous to cool and thicken even at old ages. The presence of a low velocity zone (LVZ) may be indicated between 69 and 91s but is not resolved within error. If this corresponds to the base of the lithosphere at 95 km ±50 km, this is consistent with the Plate and GDHI (geodynamic heatflow) lithospheric formation models. The 2D tomography results show the presence of a low velocity anomaly at periods from 18 s to 29 s that is ~300 km diameter located between the eastern and the western array, which coincides with the location of seamounts between the two study areas. I find that the anisotropy in the eastern region has a magnitude of 0.6 % and a consistent fast direction of NNW for nearly all periods. The western array, displays stronger anisotropy with a magnitude of 1.0 % and that is approximately E - W for short periods below 29 s and shifts to NNW at periods above 33 s. Anisotropy in both regions at short periods is consistent with the fossil spreading direction in each arm of the magnetic bight.

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