Validity of the stress inversion of orientation data from a dike swarm with a radial-parallel pattern transition

The fitting of a stress trajectory pattern predicted by a 2D elastic model to an observed radial-parallel transitional dike pattern determines the orientation of far-field stress and pressure in the central magma chamber. We found such a transitional pattern in the Amakusa region, western Japan. On the other hand, the 3D dike orientations can be inverted using Bingham distributions to determine stress axes, stress ratios, and driving pressure ratios. From the Amakusa data, the 2D analysis indicated roughly N-S extensional far-field stress. The 3D analyses conducted in the parts of the dike swarm yielded horizontal maximum stress axes roughly parallel to the local stress orientations predicted by the 2D model. Subsets obtained in distant areas from the central magma chamber indicated a normal faulting stress regime with roughly N-S extension. So, both the 2D and 3D approaches led to consistent results. The 3D analyses indicated the declining frequency of the injection of highly overpressured magmas with distance from the chamber. Thus, we conclude the 3D analysis is useful to study dike swarms with transitional patterns. The paleostress determined from the Amakusa dikes suggests that the Shikoku basin subducted under Kyushu including the Amakusa region in the early middle Miocene.

Automated summary

By analyzing the stress trajectories using 2D and 3D models, the orientation of stress and pressure in magma chambers was determined. The analysis results led to consistent conclusions regarding far-field stress orientation and pressure orientation.