Discordant contrasts of P- and S-wave speeds across the 660-km discontinuity beneath Tibet: A case for hydrous remnant of sub-continental lithosphere

Using high-resolution, triplicate shear-waveforms recorded by broadband seismic arrays, we show that a corresponding anomaly of high S-wave speed (VS) is absent where an anomaly of high P-wave speed (VP) was recently recognized in the transition zone of the mantle (TZ) beneath central Tibet. A likely cause of the discrepancy between anomalies in VP and VS is a minor amount of water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent-continent collision, the Tibetan mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. Hydration of the sub-continental lithospheric mantle (SCLM) provides a natural mechanism to facilitate ductile deformation, so rapid thickening of the SCLM, which would have been hindered by advection of cold materials, can take place. Convective instability would then lead to removal and sinking of thickened, cold SCLM, leaving a remnant in the TZ detectable only in VP. This interpretation not only is consistent with previous reconstruction of the SCLM in Tibet based on several types of independent data, but also provides a new pathway for water to enter and be stored in the TZ.