2022 Volume 63 Issue 1 Pages 2-12
The double ridges in the Kanmuriyama area, central Japan, were formed by deep-seated gravitational slope deformation（DSGSD）. The sediments deposited between the ridges consist of three units:（a）alternating carbonaceous mud and layers rich in plant remains,（b）light-gray mud, and（c）orange conglomeratic mud, in descending order. Unit（c）is interpreted to be basal conglomerate above the basement unconformity. The carbon-14 accelerator mass spectrometry（AMS-14C）ages of wood fragments, and the Kikai-Akahoya（K-Ah）tephra that erupted at about 7,300 cal BP are embedded in unit（a）, indicate that the accumulation rate of the sediments is about 0.25 mm/year, and the double ridges are estimated to have formed over about 11 ka. The results of the electrical resistivity tomography（ERT）survey suggest that the sedimentary basin is wedge-shaped, thinning to the east, and was formed by rotational sliding along the estimated gravitational fault dipping steeply to the east and limiting the western side of the basin. The horizon of the K-Ah tephra, with a ground surface of 7,300 years ago, is flat and abuts on the basement rocks. Therefore, it is inferred that the DSGSDs stopped major activity before 7.3 ka and the slope is now stable. The trigger of the DSGSD activity is most probably the climate change from cold/dry to temperate/wet conditions at the end of the last glacial period. Similar slope instabilities at the time of deglaciation, accompanied by oversteepening, debuttressing and unloading, were reported from mountain ranges worldwide. The lithological change from unit（b）to（a）might also have resulted from the vegetation change during this climatic shift.