Seismic Evidence for Hydration of the Mantle Transition Zone Beneath Tibet and Its Implications for Water‐Rich Mantle Upwelling

Abstract Seismic studies have suggested widespread thickening of the mantle transition zone (MTZ) beneath the Tibetan Plateau, which is commonly attributed to the foundering of cold lithosphere. However, the spatial pattern of MTZ thickening does not consistently align with the distribution of high‐velocity anomalies, suggesting that other factors, such as hydration, also influence MTZ structure. In this study, we employ a dense broadband seismic array combined with existing regional deployments to obtain improved receiver function images of the MTZ beneath Tibet. After correcting for upper mantle heterogeneity using a recent high resolution tomographic model, we combine MTZ thickness anomalies with mean S‐wave velocity perturbations to quantify lateral variations in temperature and water content. Our results show that the MTZ beneath the plateau contains on average at least 0.31 wt.% H 2 O and is underlain by a low‐velocity layer (LVL) immediately above the 410‐km discontinuity. These features exhibit a pronounced north‐south asymmetry: the northern MTZ is relative warm and water‐rich, whereas the southern MTZ is cooler and drier but overlain by a more extensive LVL. We propose that piece of cold lithosphere sank into the southern MTZ has displaced water‐rich mantle material from the MTZ into the upper mantle, leading to dehydration above the MTZ, formation of the LVL, and giving rise to a water‐rich mantle upwelling. These processes likely played a key role in generating Miocene porphyry copper deposits in the southern Tibetan Plateau.