5:15 PM - 6:45 PM
[HCG20-P03] Study on relationship between fracture frequency and lithofacies of granite –a case study at the Toki Granite, central Japan–
Keywords:granite, fracture, deep geological disposal
Fractures in the crystalline rock such as granite can act as fluid flow and contaminant transport conduits. Therefore, understanding of the fracture characteristics (population, orientation, infilling minerals and so on) is an important subject for the disposal of high-level nuclear waste.
The Toki granite is one of the Late Cretaceous plutonic bodies in Japan and is a nearly circular stock, approximately 14 × 12 km2 in areal extent, comprising a zoned pluton with three lithofacies grading from muscovite-biotite granite (MBG) at the margin through hornblende-biotite granite (HBG) to biotite granite (BG) in the interior. Nineteen deep boreholes, ranging from about 500 to 1300 m length, were drilled into the Toki granite. Data on fracture characteristics were obtained by the borehole investigations. The fracture frequencies often show an abrupt change around the boundary between lithofacies (Yuguchi et al., 2012).
Though the formation of open-fracture results in an increase of volume of rock mass, it is hard to envisage a geological phenomenon which causes a volume increment under compressional stress field. On the other hand, phase transition from high-temperature to low-temperature quartz causes a volume deduction. Such phenomena during magma cooling process could affect the fracture formation, and generate different fracture frequency among lithofacies. We compared fracture data with geological and petrographical data of each lithofacies to elucidate important geological event and/or process for fracture formation in this study.
Fracture frequency is relatively large in MBG to HBG and BG. This characteristic causes an abrupt change of fracture frequency around the boundary between MGB and HBG. Fracture frequency of MBG is relatively large in the northwestern part to the western part and the southern part of the Toki granitic rock body, and relatively small in the northeastern part.
Origin of MBG is considered to be assimilation of crustal host rock or simultaneous intrusion of different magma (Yuguchi et al., 2010). Host rock of the Toki granite is pelagic sediment of the Mino Terrane in the northwestern to the western parts the Toki granitic rock body, while pelagic sediment of the Mino Terrane and the Nohi rhyolite in the northeastern part. MBG is relatively thin in the western part, while relatively thick in the eastern part. The difference of the host rock may generate difference of fracture frequency. But the distribution of the lithofacies and fracture frequency don’t coincide. This fact indicates a possibility that the different fracture frequency was generated by local factor. To understand change of fracture frequency around the boundary between lithofacies, it is necessary to discuss not only the lithofacies but also local factor.
This work is supported by JSPS KAKENHI Grant Number 20K05410.
Reference
Yuguchi et al. (2012) Eng. Geol., 149-150, 35–46.
Yuguchi et al. (2012) JMPS, 39, 50–70.
The Toki granite is one of the Late Cretaceous plutonic bodies in Japan and is a nearly circular stock, approximately 14 × 12 km2 in areal extent, comprising a zoned pluton with three lithofacies grading from muscovite-biotite granite (MBG) at the margin through hornblende-biotite granite (HBG) to biotite granite (BG) in the interior. Nineteen deep boreholes, ranging from about 500 to 1300 m length, were drilled into the Toki granite. Data on fracture characteristics were obtained by the borehole investigations. The fracture frequencies often show an abrupt change around the boundary between lithofacies (Yuguchi et al., 2012).
Though the formation of open-fracture results in an increase of volume of rock mass, it is hard to envisage a geological phenomenon which causes a volume increment under compressional stress field. On the other hand, phase transition from high-temperature to low-temperature quartz causes a volume deduction. Such phenomena during magma cooling process could affect the fracture formation, and generate different fracture frequency among lithofacies. We compared fracture data with geological and petrographical data of each lithofacies to elucidate important geological event and/or process for fracture formation in this study.
Fracture frequency is relatively large in MBG to HBG and BG. This characteristic causes an abrupt change of fracture frequency around the boundary between MGB and HBG. Fracture frequency of MBG is relatively large in the northwestern part to the western part and the southern part of the Toki granitic rock body, and relatively small in the northeastern part.
Origin of MBG is considered to be assimilation of crustal host rock or simultaneous intrusion of different magma (Yuguchi et al., 2010). Host rock of the Toki granite is pelagic sediment of the Mino Terrane in the northwestern to the western parts the Toki granitic rock body, while pelagic sediment of the Mino Terrane and the Nohi rhyolite in the northeastern part. MBG is relatively thin in the western part, while relatively thick in the eastern part. The difference of the host rock may generate difference of fracture frequency. But the distribution of the lithofacies and fracture frequency don’t coincide. This fact indicates a possibility that the different fracture frequency was generated by local factor. To understand change of fracture frequency around the boundary between lithofacies, it is necessary to discuss not only the lithofacies but also local factor.
This work is supported by JSPS KAKENHI Grant Number 20K05410.
Reference
Yuguchi et al. (2012) Eng. Geol., 149-150, 35–46.
Yuguchi et al. (2012) JMPS, 39, 50–70.