Seismic imaging of the lithosphere-asthenosphere boundary beneath the Pacific including anisotropic constraints from SS waveform modelling

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, neither the depth nor the defining mechanism of the LAB are globally 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, although several recent studies have imaged discontinuity structure beneath much of the Pacific using receiver functions from buried borehole seismometers and land stations located at ocean-continent margins, SS precursors, multiple bounce S-waves, ScS reverberations, and surface waves. Overall, there is much agreement in the depth of the LAB where it is reported. Sensitivity arguments and different wavelengths sampling structure may explain some apparent discrepancies. However, interpretations vary and an exact age-depth relationship has not been clearly identified, nor has a defining mechanism for the LAB. In addition, discrepancies in the presence vs. the absence of the boundary exist, which suggest greater complexity. Here we test the possibility that discrepancies in existence of a strong sharp discontinuity are caused by anisotropic structure. We stack SS waveforms with bouncepoints in the central Pacific in 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 that is best described by anisotropy in the shallow lithospheric layer, aligned roughly in direction of plate motion. We suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate, and must be better analyzed to determine the age-depth dependence and defining mechanism of the oceanic lithosphere.