Three-dimensional fracture distribution in relation to local cooling in a granitic body: An example from the Toki granitic pluton, Central Japan

This paper describes a spatial distribution of fracture frequency (fracture number per unit interval) in the Toki granitic pluton, Central Japan, and their evaluation with respect to cooling behavior after solidification of the pluton. Three-dimensional distribution of fracture frequency of all fractures (all-angle fractures), based on borehole television data collected from 19 boreholes, shows that 1) the central region of the granite has a high fracture frequency and 2) fracture frequency decreases with decreasing elevation measured from the mean sea level, with the exception of the west side of the body. The development of sub-solidus reaction textures reflects the annealing duration of the pluton after solidification. The extent of sub-solidus reaction textures was measured at two neighboring positions, and the difference was divided by the distance between the two positions. The resulting quantity is here defined as a “local cooling indicator (LCI)”. We observed a related pattern between the distribution of fracture frequency and LCI; the high LCI region corresponds to the region with a high frequency of all fractures, and vice versa. This relation indicates that fracture genesis has been mostly controlled by “local cooling” after solidification which caused thermal stress. Thus, the three dimensional pattern of LCI is a potential diagnostic tool for obtaining the spatial distribution of all fractures.