Superplume Project: Towards a new view of whole Earth dynamics

Abstract

According to the theory of plate tectonics proposed in the late 1960’s, the Earth’s surface is covered by about ten rigid plates, which are generated at mid-oceanic ridges, move to oceanic trenches, and there subduct into the mantle. A theory of plate tectonics explains most geologic phenomena such as seismicity, volcanism, and magnetic striping of the ocean-floor. However, the theory of plate tectonics explains only the orogenic phenomena occurring in the uppermost several hundred of kilometers of the Earth’s interior. It is, therefore, a theory covering less than 1/10 of the Earth’s diameter (left in Fig. 1).

During the 1980’s studies of seismic tomography provided a new view of the Earth’s interior, and hence our understanding of the Earth’s dynamics has considerably improved. Beneath the southern Pacific a large-scale low-velocity region is present (e.g., Inoue et al., 1990), whereas beneath Asia a higher velocity region is present. The former region is characterized by a topographic high exceeding 1000 m (e.g., Crough, 1983) and also by several hot spots, suggesting the presence of large-scale mantle upwelling. The latter region corresponds to the area in which the largest amount of oceanic lithosphere has subducted during the last 300 Ma, suggesting the presence of mantle downwelling in central Asia (middle in Fig. 1).

Maruyama (1994) and Maruyama et al. (1994) proposed “Plume tectonics” to explain not only the superficial layer but also dynamics in the whole Earth: Subducted slabs in the western Pacific are stagnant at the 670 km discontinuity because of the endothermic phase transition and eventually collapse to form a cold mantle downwelling to the core mantle boundary (CMB). Two large-scale mantle upwellings are present in the lower mantle under the south Pacific and Africa (hot superplume). The whole-mantle scale convection controlled by the large-scale flow is called plume tectonics and should play a major role in the Earth’s dynamics. The material circulation of the superplumes should carry not only heat but also incompatible elements in each layer, which should be identifiable in the chemical signature of hotspot magma (right in Fig. 1).

The Superplume Project, funded by Japan’s Science and Technology Agency, started in 1996 to examine the above working hypothesis and establish a new view of whole Earth dynamics. The project is interdisciplinary and involves a broad range of Earth sciences: seismology, geology, ultra-high-pressure experiments, geodesy, and numerical simulation of mantle convection as described in detail below.