|
Fracture system of the late cretaceous Toki granite in Central Japan was analysed by using attributes of fractures collected both from boring-core and borehole TV images. We performed a matching process during which only fractures correlating by a one to one relationship in depth between the non-oriented cores and BTV images of the same holes, and new datasets were elaborated with more detailed information for each fracture. Attitude of overall fractures observed in the four studied boreholes ( DH-8, DH-9, DH-13, DH-14) each about 1000 meters deep, show similar orientations: a subhorizontal set which is identified in all four holes and three other subvertical sets striking mainly EW to ENE, NW-SE and NE-SW in DH-8, DH-9 and DH-14. According to their morphology, fractures are organized into a group of irregular shape called I-type and a group of planar surface shape called P-type. The group of I-type is mainly composed by subhorizontal fractures mostly without filling minerals or with poor infillings. Whereas the P-type group is represented by subvertical fractures sets commonly filled with clay minerals, calcite, pyrite and quartz. Their strikes are consistent with the orientation of the main faults around the study area namely the Tsukiyoshi fault (EW), the Suzuki fault (NNW-SSE) and the Yamada fault zone (NE-SW). Their densities increase at some levels where distinct structural features are intersected by the boreholes. Some unaltered fracture surfaces disclose the presence of slickenlines that obviously testified a shearing mode, suggesting that subvertical fractures are linked to the activities of these faults, in contrast to the I-type fractures which are supposed to be releasing-joints that accompany cooling and /or uplift of the pluton.
Keywords: Fracture analysis; Cretaceous Toki Granite; boring core; Borehole TV image
|
