A Vertically Dominated Stress State Near the Center of a Radial Dike Swarm: A Case Study From the Amakusa Islands, Western Japan

Abstract

Radial dike swarms have usually been thought to develop under a stress field in which local stress due to magma pressure in a central reservoir overprints the regional tectonic stress field. However, the three-dimensional stress state near the swarm center, particularly the contribution of vertical stress, remains poorly understood because dike patterns have generally been evaluated in map view. I addressed this issue by performing paleostress inversions using the three-dimensional orientations of the dikes on the Tomioka Peninsula in the Amakusa Islands, western Japan. A Middle Miocene dike swarm that shows a spatial transition from radial to parallel orientations over a tens of kilometers scale is exposed in the islands, with its radial center located around the peninsula. I measured three-dimensional orientations of planar intrusions along coastal exposures on the peninsula and corrected them for postemplacement tilting. The tilt-corrected dikes dip steeply and show variable strikes that define an approximately radial pattern around a granodiorite body. Paleostress inversions were performed for spatially divided subsets of the dike orientations. As a result, the inferred stress tensors consistently had subvertical maximum principal stress axes and low stress ratios, indicating a vertically dominated stress field during intrusion. The stress field could have resulted from local stresses associated with either a pressurized magma reservoir, volcanic loading from the overlying edifice, or both, although the latter probably had a stronger influence.