Variable forearc stress behind the eastern Nankai subduction zone in the last two million years

Transpressional tectonics characterizes the SW Japan arc. However, offshore seismic sections and onshore mesoscale faults indicate that the eastern part of the forearc was subject to transtensional tectonics in the Quaternary. Offshore normal faults found on the seismic profiles started their activity at ca. 1 Ma. In order to understand the stress history in the forearc, we collected fault-slip data from onshore mesoscale faults in Plio-Pleistocene sediments in the eastern SW Japan forearc. Most of the mesoscale faults are of oblique-normal in type, indicating that the area was subject to transtensional tectonics. The faults suggest that the compressional tectonic regime was succeeded by the transtensional one at 2 Ma, consistent with tectonostratigraphy in the region that folding ceased at that time. Present compressional stress followed the transtensional tectonic regime sometime in the late Pleistocene. These observations indicate the state of stress just behind the accretionary prism of the eastern Nankai subduction zone has been unstable in the last ca. 2 million years, suggesting that the forearc wedge has been at critical states in that gravitational force and basal shear traction on the wedge has been balanced. Possible factors compatible with the observed stress history include the change of subduction direction of the plate at 1 Ma, and the rapid uplift of Central Japan thereafter.