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.