The establishment of the theory of plate tectonics at the end of the 1960s provoked quantitative discussions regarding the forces acting on lithospheric plates. Subsequent studies during the early- to mid-1970s considered plate motions as rigid rotations on a spherical surface. A theoretical analysis based only on tectonic information from the Earth’s surface revealed that a candidate for the primal driving force of plate motion was “slab pull”, which may be balanced almost completely by “slab resistance”. However, because plate interiors of the real Earth have finite effective viscosity and are part of the cold thermal boundary layers involved in mantle convection, they should move with an element of internal deformation rather than perfectly rigid motion. A recent numerical simulation of 3-D spherical mantle convection revealed that the breakup of Pangea, subsequent continental drift, and the present-day continental distribution, could be acheived by planetary-scale mantle flow. Large-scale lateral mantle flow is inferred to have originated from a high-temperature anomaly region beneath Pangea due to a supercontinental thermal insulation effect, rather than by mantle upwelling flow from a “superplume”, and subduction of cold boundary layers is inferred to have spontaneously developed in the North Tethys Ocean during the early stages of the breakup of Pangea. The present results, combined with other numerical simulation results and seismological evidence from a recent sub-seafloor structure survey, indicate that the “(continental) mantle drag force”, enhanced by mantle flow beneath the continental/oceanic plates, could be the primal driving force of plate motion and continental drift. This possibility raises new questions about whether the slab-pull force or mantle-drag force is the primal driving force for plate motion and continental drift.
Until now, there has been no consensus over the stratigraphic correlation between the western and eastern parts of the Hinagu Belt in the Yatsushiro Mountains, Kyushu. Therefore, we examined the stratigraphy and structure of the Pre-Sotoizumi Group in the central part of the Hinagu Belt, with a particular emphasis on clarifying the stratigraphy and distribution of the Hachiryuzan, Kesado and Imaizumigawa formations. The Kawaguchi Formation was identified as the oldest formation in the study area, followed by the Hachiryuzan, Kesado and Imaizumigawa formations. The distribution of the Kesado and Imaizumigawa formations was confirmed in the eastern part of the study area, but the distribution of the former could not be recognized in the western part. The relationships between the Hachiryuzan and Kesado formations, and between the Hachiryuzan and Imaizumigawa formations were confirmed as being conformable and disconformable, respectively, as identified in previous reports. In contrast to previous reports, however, field evidence shows that the Kesado and Imaizumigawa formations are separated by a clino-unconformity. It is concluded that these two formations were folded during the Barremian-Aptian, prior to the deposition of the Imaizumigawa Formation.