3-D MAPPING OF THE REMNANT SLABS THE “PALEO-PACIFIC PLATE” IN THE LOWER MANTLE UNDER EAST ASIA AND IMPLICATIONS FOR PLATE RECONSTRUCTION OF THE MESOZOIC EAST ASIA

It is often suggested that Paleo-Pacific plate subduction dominated the Jurassic-Cretaceous geology of East Asia, creating a 6000-km-long belt of magmatism, deformation, and metamorphism from Far East Russia to Borneo. Nevertheless, the geometry, kinematics, and even nomenclatures of the Paleo Pacific are highly controversial, due to the fact that it had entirely subducted by Eocene. To better reconstruct the plate tectonics of the East Asia region, we use seismic tomography models to map the subducted slabs in 3-D by interpreting high velocity anomalies and tracing their midslab surfaces.

Besides the previously-recognized slabs at ca. 1100-km or shallower depths, which are associated with the Pacific, Philippine Sea, Eurasia, Australia, and East Asia Sea plates, we identify three major slabs in the lower mantle, namely: Izanagi, Hunan, and Taipei. The Izanagi slab is a N35E-trending, steeply west-dipping, 2700-km wide slab wall. It is located at ca. 1100-2100 km depths beneath Khabarovsk and Amur of Far East Russia and the Great Xing’an Range in Northeast China. To the south, the Hunan slab is separated by a near-vertical gap from the Izanagi. It strikes N15E and dips vertically. The top is at ca. 1450 km depth, and the bottom merges into the “slab graveyard” at the base of mantle. Its width is 3500 km at deeper depth, but decreases upward. If projected to surface, this slab ranges from the Taihang Mountains in North China, through Hunan Province of South China, to northern Vietnam. Unlike the two NNE-trending slab walls, the Taipei slab dips moderately to the south, and strikes ENE at shallow depths (1050-1500 km) and ESE at 1600-2000 km depths. It lies under the Taiwan Island and the Philippine Sea with a minimal width of 2200 km. The so-called Paleo-Pacific consists of three slabs with different history. Their geometry and spatial relationship provide independent evidence for East Asian tectonics, including: 1) extensive, Jurassic-Early Cretaceous arc magmatism along the entire margin, 2) subduction cessation in SE China in early Late Cretaceous, 3) existence of a slab tear followed by a sub-parallel ridge subduction, 4) rotational, retreating trench during the Taipei slab subduction, 5) exotic terrane accretion, possibly the Okhotomorsk Block, and 6) opening of the East Asia Sea by trench retreat and backarc extension.