Upper mantle seismic shear discontinuities of the Pacific

Shear wave impedance discontinuities are inventoried for eight paths connecting circum-Pacific earthquakes with seismic stations on Hawaii and Oahu. In addition to the transition zone discontinuities, we observe a consistent impedance decrease at a depth of ∼80 km that marks the transition from the fast seismic lid to the low-velocity zone. The interval over which this transition occurs is less than 30 km. The requisite impedance decrease, ascribed almost entirely to diminished velocities in the low-velocity zone, exceeds predictions for peridotite chemistries of appropriate lithospheric age. In all cases, the transition is better matched by lithosphere of roughly half of the true age. Four paths show clear evidence of the X discontinuity, an impedance increase of 3-8% near 300 km depth. The only viable explanations would require extensive eclogite or basalt-depleted mantle in the upper mantle of the western Pacific. The impedance contrast of the 410-km discontinuity, which depends on the modal fraction of mantle olivine, does not signal the presence of either, although a mixture of the two remains a possibility. We find no evidence of a low-velocity layer atop of the 410-km discontinuity. If present, it is either consistently thin (≤15 km), highly variable in thickness (topography in excess of 60 km) or has little impedance contrast (<2-3%) with overlying mantle. The apparent absence of melt atop the 410-km discontinuity, an intermittent and weak 520-km discontinuity, and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central Pacific.