Arc‐parallel extension in preparation of the rotation of southwest Japan: Tectonostratigraphy and structures of the Lower Miocene Ichishi Group

Opening of the Japan Sea back arc basin was accompanied by extensional tectonics in the drifting southwest Japan arc. Various trends of Early Miocene grabens in the arc suggest multi‐directional rifting, which necessarily involved strike‐slip components of some of basin‐margin faults. However, such components are not well understood. In this work we conducted a field survey in the Early Miocene Ichishi basin on the northern side of the Median Tectonic Line, central southwest Japan. We found that the basin was a compound of grabens that were formed along normal and sinistral strike‐slip faults, the latter of which had northeast–southwest trends. The block faulting in this phase produced basement highs between sub‐basins, which were filled with the lower part of the Ichishi Group. We found a low‐angle angular unconformity at a middle horizon in the group, with which we define the upper and lower part of the group. The upper part onlapped both the basement highs and the lower part. It means that the transtensional basin formation ceased sometime between 18 and 17.5 Ma in the Ichishi area. The Ichishi basin turned subsequently into a sag basin subsided due to normal faulting probably along the Nunobiki‐sanchi‐toen fault zone. The transtension and the basin sag were driven by ENE–WSW extensional stress. This arc‐parallel extension produced grabens various areas including Ichishi in the Early Miocene. The extensional deformation was eventually localized to the deep rift along the Fossa Magna to make the lithosphere under southwest Japan decoupled from that under northeast Japan. The decoupling allowed the rapid rotation of southwest Japan from ~17.5 Ma. The cluster of those grabens around the Ise bay probably determined the southeastern margin of the Kinki triangle.