Field Observation of Cooling Joint System in the Quaternary Takidani Pluton, Japan

Abstract

Due to rapid uplift in the Quaternary, the natural fracture network in the Takidani Pluton can now be observed in outcrops. Distinctive features characterising fracture networks in the neo-granitoid body were recognised during field observations; preferred orientations were identified but no displacement on fractures were observed. These fractures have the general characteristics of joints and cut across each other to form blocks of parallelepiped shape. Strike and dip of fracture surfaces were measured and show that fracture orientation changes progressively from the margin of the body towards the centre of the pluton. Joint spacing, measured as a dimension of respective joint blocks, also increases closer to the centre of the pluton. The characteristics of the fracture network, particularly the preferred orientations and joint spacing (with no displacement), indicate that the formation mechanism of fractures in the pluton was due to thermal contraction, rather than shearing, in a regional stress field. Reconstruction of the plutonic body is possible by plotting the perpendicular direction of joint surfaces for each outcrop.