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.

中文翻译：

---

为日本西南旋转做准备的弧平行伸展：下中新世 Ichishi 群的构造地层和构造

日本海后弧盆地的开放伴随着日本西南弧漂移的伸展构造。弧中早中新世地堑的各种趋势表明多向裂谷，这必然涉及一些盆地边缘断层的走滑成分。然而，这些组件还不是很清楚。在这项工作中，我们在日本西南部中部构造线北侧的早中新世 Ichishi 盆地进行了实地调查。我们发现盆地是沿着正常和左旋走滑断层形成的地堑复合体，后者具有东北-西南走向。这一阶段的断块断层在子盆地之间产生了基底高点，这些盆地充满了 Ichishi 群的下部。我们在群的中间层位处发现了一个小角度角不整合，我们以此来定义群的上部和下部。上部与地下室高点和下部重叠。这意味着在 Ichishi 地区在 18 到 17.5 Ma 之间的某个时间停止了张力盆地的形成。由于正断层可能沿布引山地园断层带，市志盆地随后转变为凹陷的凹陷盆地。ENE-WSW 拉伸应力驱动了横张和盆地凹陷。这种弧平行延伸产生的地堑包括早中新世的 Ichishi 在内的各个地区。拉伸变形最终局限在大窝深裂谷中，使日本西南部的岩石圈与日本东北部的岩石圈脱钩。解耦使日本西南部从~17.5 Ma 开始快速旋转。伊势湾周围的地堑群可能决定了近畿三角的东南边缘。