5:15 PM - 6:30 PM
[SCG50-P10] Transition from radial to parallel dike swarm in Amakusa, Japan and their paleostress analyses
Keywords:stress, dike, sill
Dikes in the Amakusa Islands were believed to have NNE trends, suggesting their intrusion accompanied by the initial rifting of the Okinawa Trough (Yamamoto, 1991). The sheet intrusions intruded to the Cretaceous and Paleogene sedimentary rocks, and the timing of intrusion was constrained to be 14–19 Ma depending on the radiometric ages of the intrusive rocks (Shibata and Togashi, 1975; Nagao et al., 1992; Hamasaki, 1996; Ohira et al., 2012). We observed 243 sheet intrusions in the islands and found that the NNE trending dikes were parts of a radial dike swarm. We also found that the radial pattern transformed laterally to a parallel one.
We took two approaches to estimate the paleostress(es) during the intrusion. First, to estimate the direction of σHmax-axis during intrusion, a two-dimensional elastic model which involved the transition (e.g., Odé, 1957) was fitted to the distribution and trends of the sheet intrusions. The model assumed an elastic body with an internally pressured circular hole subjected to a bi-axial far field stress. This analysis showed the regional E-W trending σHmax-axis.
Second, orientation data from the sheet intrusions were analyzed by the method of Yamaji and Sato (2011), which fits a mixed Bingham distribution to the orientations. This method was applied to the data set that consisted of 26 data taken from an area at the distances from 30 to 40 km form the center of the radiating system. The area existed within the parallel system. As a result, we obtained Stresses A and B of normal and strike-slip faulting regimes. Their σ3-axes had N-S and ENE trends, respectively. Since the latter was obtained from sills in steeply dipping, well stratified sedimentary rock, we interpreted that Stress B was an artifact resulted from the mechanical weakness of the bedding. On the other hand, the principal axes of Stress A were approximately parallel to those determined by the two-dimensional elastic model. Therefore, Stress A was probably the regional stress at the time of dike intrusion. Stress A had the stress ratio of 0.40.
Magma pressure may have had temporal and spatial variations. The stress analysis yielded the DPI of 0.09: DPI (driving pressure index) is the 95th percentile of driving pressure ratios of the magmas (Faye et al., 2018). The orientation data of intrusions in an area from 2 to 10 km from the radiating center had two clusters indicating DPIs of 0.38 and 0.60 (Ushimaru and Yamaji, 2020), a few times larger than the DPI determined from the dikes far from the center. This difference is consistent with the picture that magma pressure declines with the distance from the center of a radiating dike system.
We took two approaches to estimate the paleostress(es) during the intrusion. First, to estimate the direction of σHmax-axis during intrusion, a two-dimensional elastic model which involved the transition (e.g., Odé, 1957) was fitted to the distribution and trends of the sheet intrusions. The model assumed an elastic body with an internally pressured circular hole subjected to a bi-axial far field stress. This analysis showed the regional E-W trending σHmax-axis.
Second, orientation data from the sheet intrusions were analyzed by the method of Yamaji and Sato (2011), which fits a mixed Bingham distribution to the orientations. This method was applied to the data set that consisted of 26 data taken from an area at the distances from 30 to 40 km form the center of the radiating system. The area existed within the parallel system. As a result, we obtained Stresses A and B of normal and strike-slip faulting regimes. Their σ3-axes had N-S and ENE trends, respectively. Since the latter was obtained from sills in steeply dipping, well stratified sedimentary rock, we interpreted that Stress B was an artifact resulted from the mechanical weakness of the bedding. On the other hand, the principal axes of Stress A were approximately parallel to those determined by the two-dimensional elastic model. Therefore, Stress A was probably the regional stress at the time of dike intrusion. Stress A had the stress ratio of 0.40.
Magma pressure may have had temporal and spatial variations. The stress analysis yielded the DPI of 0.09: DPI (driving pressure index) is the 95th percentile of driving pressure ratios of the magmas (Faye et al., 2018). The orientation data of intrusions in an area from 2 to 10 km from the radiating center had two clusters indicating DPIs of 0.38 and 0.60 (Ushimaru and Yamaji, 2020), a few times larger than the DPI determined from the dikes far from the center. This difference is consistent with the picture that magma pressure declines with the distance from the center of a radiating dike system.