The Pacific lithosphere-asthenosphere boundary: Seismic imaging and anisotropic constraints from SS waveforms

The lithosphere-asthenosphere boundary (LAB) separating the rigid lid from the underlying weaker, convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics. However, the exact depth and defining mechanism of the LAB interface remain poorly understood. The ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70% of Earth's surface area and have a relatively simple geological history. Seismically imaging the oceanic LAB at high resolution has proved challenging. Yet, several studies have recently increased resolution with provocative results. We summarize recent imaging of discontinuity structure beneath much of the Pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean-continent margins, SS precursors, and waveform modeling of multiple phases including multiple bounce S waves, ScS reverberations, and surface waves. Overall, there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the LAB. Some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths. Yet, when the results are considered together, no single age-depth relationship is illuminated. There are also puzzling discrepancies in where the discontinuity is detected, which again suggests greater complexity. Here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure. We stack SS waveforms with bounce points in the central Pacific into azimuthal bins. We use two methods, one that inverts for discontinuity structure based on subtle variations in the character of the SS waveform, and another that considers SS at higher frequency. We find azimuthal variation in the amplitude of the waveform, including a polarity reversal. We suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate, and must be carefully considered to constrain the age-depth dependence and defining mechanism of the oceanic lithosphere.