Convective coupling between ductile lower crust and upper mantle, and its tectonic implications

The dynamic response of the crust and upper mantle to asthenospheric convective flow related to hot mantle diapiric upwelling is numerically evaluated in order to examine the implications for geological phenomena such as rifting, back‐arc spreading, sedimentary basin and mountain building. For island arc areas such as the Japanese Islands and for ridge subducting areas such as the Basin and Range region, where both lower crust and upper mantle may behave as ductile layers in some evolutionary stages, the mechanical coupling between convection in the two layers may play an important role in the mantle dynamics related to these geological phenomena. Significant surface response to mantle diapiric upwelling occurs even when the whole crust effectively works as an elastic layer. The magnitude of surface uplift predicted for this crustal rheological state depends on the thickness of the lithosphere and on the spatial scale of the mantle diapir. However, the effective viscosity of the lower crust may decrease with increasing duration of diapiric upwelling. If the effective viscosities of the lower crust and asthenosphere are 10¹⁹‐10²¹ Pa s, the asthenospheric convective flow induced by a mantle diapiric upwelling with a radius about 50 km, a scale of feature which has been seismically observed in the upper mantle beneath the Japanese Islands, may squeeze out the lower crustal material laterally. This would be followed by maximum surface subsidence of about 300–500 m and by upward migration of the Moho discontinuity by up to 3–5 km on a geological time‐scale of 5–50 Ma. Thus, the coupling between the convection in the lower crust and upper mantle is mechanical, and the lower crustal material is dragged along by the shear stresses operating at the base of the lower crust. In this process, the space originally occupied by lower crustal material is replaced by mantle material, leading to thinning of the lower crust. Thus, in some geological situations having a hot mantle plume beneath the crust, a change of tectonic style can be expected as a result of a change of the state of convective coupling between the lower crust and upper mantle. Geological phenomena associated with this convective coupling may also depend on the geometry of the region surrounding the mantle diapiric upwelling. For example, the subducted slab in island arc areas will operate as a barrier to the lateral movement of lower crustal material. Thus, accumulation of lower crustal material, referred to as lower crustal megalith, may occur at the edge of the overriding plate. The overpressure caused by the accumulation of this megalith may be a crucial factor in determining the coupling state between the subducted slab and island arc, and may play an important role in and/or work as a trigger in the evolutionary histories of the island arc and related back‐arc spreading.