Unstable forearc stress in the eastern Nankai subduction zone for the last 2 million years

Transpressional tectonics characterizes the SW Japan arc. However, we will show in this article that offshore seismic profiles and onshore mesoscale faults indicate that the eastern part of the forearc was subject to transtensional tectonics since ca. 2.0 Ma. Offshore normal faults imaged on the profiles run parallel to the Nankai Trough, and started activity at 1.0 Ma, but transtensional tectonics commenced the onshore area earlier. In order to understand the stress history in the forearc region, we collected fault-slip data from onshore mesoscale faults in Plio-Pleistocene sedimentary rocks in the Kakegawa area at the northeastern extension of the offshore normal faults. Most of the mesoscale faults are oblique-normal, indicating that the area was subject to transtensional tectonics. The faults suggest that the compressional tectonic regime was followed by the transtensional one at 2.0 Ma, in agreement with regional tectonostratigraphic data, which indicate that folding ceased at that time. Present compressional stress followed the transtensional tectonic regime sometime in the late Pleistocene. Transtensional or extensional tectonic zone shifted from the Kakegawa area to the offshore region.These observations indicate that the state of stress just behind the accretionary prism of the eastern Nankai subduction zone has been unstable in the last 2 million years, suggesting that the forearc wedge has been at critical state in that gravitational force and basal shear traction on the wedge have been balanced, but the forearc tectonics has been susceptible to small perturbations. Possible factors compatible with the observed stress history include the change of subduction direction of the plate at 1.0 Ma, and the rapid uplift of Central Japan thereafter.