Roles of mantle diapir and ductile lower crust on island-arc tectonics

Abstract

In order to examine the relationships between surface-geological phenomena in island-arcs and lower-crustal and upper-mantle dynamics, crustal and upper-mantle responses induced by upper-mantle density heterogeneities with magnitudes expected from the seismic tomography beneath the northeastern Japan arc have been evaluated. According to the rheological model with a low-viscosity lower crust and low-viscosity uppermost mantle, low-density lower-crustal material above mantle diapiric upwelling is mechanically dragged along by asthenospheric shear stresses, and then lower-crustal material flows from the back-arc corner to the fore-arc corner. The surface subsidence for this process is about 300 m on a geological time scale of 5–50 My, and the formation of a sedimentary basin with about the same lateral scale as the mantle diapir can be expected in the area above the mantle diapir (stage “S”). The subducted slab in island-arc will operate as a barrier to the lateral movement of lower-crustal material. Accumulation of low-density lower-crustal material may occur at the edge of the overriding plate during this process. At a critical stage of the pressure in the mantle wedge of fore-arc region, high-temperature lower-crustal material may then be exposed along the tectonic line (stage “E”). As the result of discharging lower-crustal material along the tectonic line, the pressure in the lower crust will be decreased and accelerated subsidence accompanied by volcanism is expected in the sedimentary basin. Cretaceous geological phenomena in Southwest Japan may be explained by this physical mechanism, its linked surface crustal movement and the flow of lower-crustal material. A relatively slow deposition in the period of about 140-120 Ma observed in the Kanmon sedimentary basin may be interpreted as the geological representation of stage “S”. From about 110 Ma, the Kanmon sedimentary basin was supplied with a large quantity of volcanic sediments, and simultaneously expanded and suddenly subsided. At about the same time, low-pressure—high-temperature metamorphism and granitic activity (Ryoke Belt) occurred along the areas to the south of the Kanmon Group. It may be possible to interpret these geological phenomena along the Median Tectonic Line (MTL) as the surface-geological events corresponding to stage “E”. It is, however, stressed that the time and spatial scale for this coupling is determined by the rheological structure of the lower crust and upper mantle and by the scale of mantle diapir.