Abstract
It is controversial how and when the Japan Sea backarc basin opened. Onshore geology is an important clue for this issue. It is believed that arc-perpendicular extension was accompanied by the opening, and that SW Japan was separated and drifted from the Eurasia as a coherent block. However, there are Early Miocene grabens trending not only parallel but also perpendicular to the arc, suggesting that the crust under present SW Japan was broken into blocks. We studied the Lower Miocene geology of the Wajima area, northern Noto Peninsula, where syn- and postrift sedimentary rocks were uplifted by Late Miocene folding. We found that fan and fan delta environments were dominant in the late Early Miocene in the area. Faults with map- and meso-scales in this horizon indicate NE-SW or E-W extensional stress. Overlying Middle Miocene offshore sediments were not affected by the extensional tectonics, suggesting that the extensional tectonics had terminated by the end of the Early Miocene. The extensional orientation was parallel to the arc. This is concordant with the orientation suggested by Early Miocene grabens to the west of the peninsula, but is perpendicular to that in the northeastern part of the peninsula. These observations suggest that SW Japan was broken into polygonal blocks to drift from Eurasia.
