Spatial transition from radial compression around fluid supply source to regional stress; An Example of clastic dikes intruding into the Miocene Tanabe Group

安邊 啓明   

Abe, Noriaki

Dike orientation has been used to estimate regional stress as an indicator for tectonics. A periphery of the fluid supply source, such as a magma chamber and mud diapir, is suitable for estimating stress because of a greater number of dike intrusions. However, stress changes locally due to radial compression, and it is expected to establish the method for distinguishing the effects of regional and local stresses around the fluid supply source. Mud diapirs and associated conglomerate-bearing mudstone dikes intruded at the study area in the Miocene Tanabe Group, a paleo-forearc basin deposits in southwestern Japan. To reveal the spatial stress variation, this study measured the orientation of the dikes and performed stress inversion using the mixed Bingham distribution method (Yamaji and Sato, 2011) for each sub-area of tens to hundreds of meters. The general trend of E-W minimum compressional stress axes suggests regional E-W tension stress. Minimum compressional stress axes are relatively dispersed in the southern part of the study area. The density of dike intrusion increases from north to south. These suggest that mud diapir exists in the southern part. Stress regime changes spatially, as normal faulting, maximum compressional stress axis-plunging southward, and strike-slip faulting from south to north. This pattern corresponds to the stress change suggested by the stress field model around the spherical cavity in the semi-infinite elastic body. Based on the model, this study proposed a new stress field model by summing homogeneous regional stress. Then, the parameters of the proposed model for accounting stresses detected in the study area are estimated inversely by the Markov chain Monte Carlo method. As a result, regional stress was well constrained as E-W tension normal faulting stress.