Paleoseamounts in Yarlung Zangbo suture zone reveal dynamic processes of the Early Cretaceous Neo-Tethys lithosphere

Dynamic processes of the oceanic crust of seamounts trapped in continental sutures are poorly understood. The discovery and analysis of seamount sequences in the Yarlung Zangbo suture zone allowed us to detect vertical lithospheric movements of the eastern Neo-Tethys Ocean. In this study, we used petrological, geochemical, facies, paleontological, and stratigraphic methods to study composite blocks of mafic and sedimentary rocks in the Zhongba area of Xizang, China. Radiolarian fossils indicate an (early) Early Cretaceous age. Petrofacies and geochemistry show that most basalts occur as pillow lava and are enriched in light rare earth elements and high field strength elements, which suggests an oceanic-island basalt tectonic setting. Sedimentary lithofacies are grouped into breccias, shales, cherts, and limestones (ooid and cortoid grainstone microfacies). Three types of contact between basaltic basements and caps were identified as regular, talus, and erosive. These results demonstrate that two more superposed seamounts exist in the western Yarlung Zangbo suture zone than was known previously, and they comprise at least eight seamount sequences of basalts to sediments. Cap lithofacies and seamount textures illustrate that overall, the Zhongba paleo-seamounts record multiple cycles of abrupt deepening to shallowing, with water depth variations of ∼3000 m to ∼5 m due to multiple episodes of quick subsidence and uplift of seamounts (lithospheric “ups and downs”). The model of oceanic bypass recurrence and plume rejuvenation is hypothesized to explain the dynamic subsidence and uplift. While plume activities produced the mafic basement and considerable buoyancy-related lift, oceanic crust bypassed drift, created distance from the plumes, and led to cooling, loss of buoyancy, and rapid subsidence. This model provides a new approach to interpret the evolution of oceanic crusts that have disappeared.