Late Pleistocene crustal uplift and gravity anomaly in the eastern part of Kyushu, Japan, and its geophysical implications

The Miyazaki Plain, eastern part of Kyushu, Japan, is characterized by both significant negative gravity anomalies and aseismic crustal uplifting (~1 mm/year) in the Late Pleistocene and Holocene. We examine the relationship between these two phenomena, which may provide important constraints on the interaction between the collision and/or subduction of the Kyushu-Palau Ridge and the forearc. We estimate the mass deficiency below 11-km depth by using the gravity anomalies and P-wave velocity structure of the upper crust. The onset of the load accumulation, 0.5-0.4 Ma, is inferred from the movement of the fluvial terraces considering the tephrochronology. The loading history is assumed to be a linear function of time. We evaluate the crustal rebound by assuming a viscoelastic plate deformation with an underplating load existing at 20- or 30-km depth. The predicted crustal movement for models with a lithospheric (crustal) viscosity of 10²³-10²⁴ Pa s can explain the observed altitudes of the shoreline of the marine terraces formed at the Last Interglacial of about 125 kyr BP and the middle Holocene of 5-6 kyr BP. Although we cannot restrict the origin of the buoyant body, the subduction of the Kyushu-Palau Ridge, remnant arc associated with back-arc opening of the Shikoku Basin, may be related to the buoyancy for the uplifting region examined here. On the other hand, the buoyant body off the Miyazaki Plain probably plays an important role in the interaction between the subducting oceanic slab and the overriding forearc crust. Thus, the observed lateral variation of the interplate coupling on the convergent boundary along the Nankai Trough may be attributed to the existence of the buoyant body.