Bidirectional subduction of the Bangong-Nujiang ocean revealed by deep-crustal seismic reflection profile

It is well known that the Tibetan plateau exposes a series of terranes that were drifted northward and accreted progressively onto the south margin of Eurasia resulting from ocean closure. However, the processes associated with the ocean closure remains poorly unconstrained. One such issue is whether the demise of the Bangong-Nujiang ocean between the Lhasa and Qiangtang terranes were caused by northward subduction only or both northward and southward subduction. To address this issue, we analyze high-resolution seismic reflection data along a 275-km north-south traverse across the Bangong-Nujiang suture zone. Our work shows that the Moho of the Lhasa terrane lies at 22.5 s (two-way travel time) and the Moho of the Qiangtang terrane lies at 20 s. In the Lhasa terrane, the reflection profile reveals an X-shaped structure that segmented south-dipping reflectors are truncated by continuous north-dipping reflectors. Based on the existing geological and geophysical observations and the cross-cutting relationships among seismic reflectors, we propose that the fossil relics of south-dipping reflectors with relatively weak amplitude in Lhasa terrane were created by the Mesozoic southward subduction of the Bangong-Nujiang ocean, and the north-dipping reflectors were triggered by the younger Cenozoic deformation during the India-Asia collision. In the Qiangtang terrane, our seismic reflection profile displays an antiformal structure with a set of north-dipping reflections in the lower crust, which we interpret as triggered by the northward subduction of the Bangong-Nujiang ocean. The structures as revealed by our seismic observations as a whole indicate that the closure of the Bangong-Nujiang ocean was created by bidirectional oceanic subduction.