Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

Abstract

While several mechanisms have been suggested to explain the evolution of the Tibetan Plateau, observational constraints on the deep lithospheric processes have been sparse, and previous seismic studies were mostly along profiles perpendicular to the collision front of the Indian and Eurasian plates. In this study, we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern Tibet, a process in which the entire mantle lithosphere peels away from the crust along the Moho and thus is a mechanism for rapid thinning of the lithosphere. Our P and S wave velocity models show the presence of a low‐velocity anomaly in the crust and upper mantle down to ∼300 km depth beneath a north–south trending rift zone in southeastern Tibet. This low‐velocity anomaly is situated above a tabular, high‐dipping‐angle, high‐velocity anomaly that extends into the upper mantle transition zone. The VP/VS ratio of this high‐velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt‐depleted mantle is required. These observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north–south trending rift in southeastern Tibet.