[SSS16-P17] Some outcrops along the Ide Fault, a part of the eastern margin fault zone of the Nara basin, central Hoshu.
Keywords:active fault, early Pleistocene, eastern margin fault zone of the Nara Basin, structural evolution, Neotectonics
Introduction
The author found out some outcrops in which the basement Granite thruts upon the basal part of Osaka Group, and several angular unconformities cut and overly in the basal part of Osaka Group near the Ide Fault, north end of the southern part of the eastern margin fault zone of Nara Basin.
Osaka Group near the Ide Fault
The basal part of Osaka Group includes Mushono tephra ca.2.3 Ma, and it is made up by fluvial sand and gravels, mud and sand and fresh water bioturbated lacustrine clay. Gravels in this Group areconsisted mainly by metamorphic rocks and sedimentary rocks. There is little granitic rocks derivered from hinterlands and no Koto rhyolitic gravels in this sand and gravels.
Reverse fault and unconformity in the basal part of Osaka Group near the Ide Fault
The Osaka Group overlies on the basement granit, its unconformity is 50 degree southward dipping at one outcrop in Kamikoma, Kidukawa City. Several unconformities are detectable in the basal part of Osaka Group. A fault outcrop at where basement granit thrusts up to the Osaka Group, and clear angular unconformity is shown in the basal part of Osaka Group at 1 m up from the base of Osaka group.
Significance of angular unconformity in the basal part of Osaka Group
The Ide fault have activated at least since the earliest Gelasian Period. Its initiation era is earlier than Ikoma Fault and Uemachi Fault, which are located west of the parallel to the eastern margin fault zone of the Nara basin. This means the initiation of faulting in southern part of the Kinki Triangle is not same time, but eastern one is earlier than western one. This fact would be accompanied with Neotectonism in central Japan, and with seismic risk evalution.
The author found out some outcrops in which the basement Granite thruts upon the basal part of Osaka Group, and several angular unconformities cut and overly in the basal part of Osaka Group near the Ide Fault, north end of the southern part of the eastern margin fault zone of Nara Basin.
Osaka Group near the Ide Fault
The basal part of Osaka Group includes Mushono tephra ca.2.3 Ma, and it is made up by fluvial sand and gravels, mud and sand and fresh water bioturbated lacustrine clay. Gravels in this Group areconsisted mainly by metamorphic rocks and sedimentary rocks. There is little granitic rocks derivered from hinterlands and no Koto rhyolitic gravels in this sand and gravels.
Reverse fault and unconformity in the basal part of Osaka Group near the Ide Fault
The Osaka Group overlies on the basement granit, its unconformity is 50 degree southward dipping at one outcrop in Kamikoma, Kidukawa City. Several unconformities are detectable in the basal part of Osaka Group. A fault outcrop at where basement granit thrusts up to the Osaka Group, and clear angular unconformity is shown in the basal part of Osaka Group at 1 m up from the base of Osaka group.
Significance of angular unconformity in the basal part of Osaka Group
The Ide fault have activated at least since the earliest Gelasian Period. Its initiation era is earlier than Ikoma Fault and Uemachi Fault, which are located west of the parallel to the eastern margin fault zone of the Nara basin. This means the initiation of faulting in southern part of the Kinki Triangle is not same time, but eastern one is earlier than western one. This fact would be accompanied with Neotectonism in central Japan, and with seismic risk evalution.