11:00 AM - 1:00 PM
[SVC29-P10] Validity of the stress inversion of orientation data from a dike swarm with a radial-parallel pattern transition: A study from the Miocene dike swarm in Amakusa, Japan
Keywords:magma transport, tectonic stress, magma pressure, elastic model
Spatial and orientation distributions of dikes indicate subsurface stress at the time of intrusion. Recent stress inversion techniques can determine stress axes, stress ratio, and driving pressure ratio through clustering 3D dike orientations (Jolly and Sanderson, 1997; Yamaji and Sato, 2011). Hereafter, we refer this approach as the 3D stress inversion technique.
On the other hand, the map-scale patterns of a dike swarm also indicate subsurface stress. The swarm often shows a characteristic radial-parallel pattern transition (e.g., Knopf, 1936). The transition is explained by the superposition of far-field tectonic stress and pressure from the central magma chamber (Odé, 1957; Nakamura, 1977). The 2D elastic model incorporating this idea was initially developed by Ode (1957). The model has been often used to determine paleostress orientations from dike patterns (e.g., Muller and Pollard, 1977; Baer and Reches,1991; McKenzie et al., 1992; Koenig and Pollard, 1998).
Then, what if 3D dike orientations from a radial dike swarm are inverted? Are the solutions useful? We mapped 250 felsic planar intrusions in the Amakusa region, and found a radial-parallel pattern transition (Ushimaru & Yamaji, 2022). The 3D data sets from the parts of the radial part are useful to answer the questions, and those from the parallel part indicate the back-ground, far-field stress. Thus, the intrusions provided us with a rare opportunity to answer those questions.
To this end, we analyzed the orientation data of the Amakusa dike swarm by two approaches. First, theoretical stress trajectory pattern predicted by the 2D elastic model of Ode (1957) was fitted to the trends and spatial distribution of the dikes to determine far-field stress. Second, the 3D stress inversion technique of Yamaji and Sato (2011) was applied to the 3D dike orientations. The 3D data were partitioned into subsets according to the locations where they were obtained in the field.
As a result, we had consistent results from the two approaches. That is, the local σHmax orientations determined by the 3D stresses were roughly parallel to the stress trajectories inferred from the 2D elastic model. In addition, the technique can evaluate the frequency of highly overpressured magmas decreased with the distance from the central magma reservoir. The ratio of magma pressure to principal stress showed a negative correlation with distance from the central magma chamber. This result is consistent with the assumption of the 2D analysis.
The results of this study demonstrate the validity of the 3D stress inversion technique for radial and parallel dike swarms. The technique can determine 3D local stress conditions in parts of both radial and parallel dike swarms. The coincidence of 2D and 3D analyses indicate the validity of the assumptions of both the two analyses.
On the other hand, the map-scale patterns of a dike swarm also indicate subsurface stress. The swarm often shows a characteristic radial-parallel pattern transition (e.g., Knopf, 1936). The transition is explained by the superposition of far-field tectonic stress and pressure from the central magma chamber (Odé, 1957; Nakamura, 1977). The 2D elastic model incorporating this idea was initially developed by Ode (1957). The model has been often used to determine paleostress orientations from dike patterns (e.g., Muller and Pollard, 1977; Baer and Reches,1991; McKenzie et al., 1992; Koenig and Pollard, 1998).
Then, what if 3D dike orientations from a radial dike swarm are inverted? Are the solutions useful? We mapped 250 felsic planar intrusions in the Amakusa region, and found a radial-parallel pattern transition (Ushimaru & Yamaji, 2022). The 3D data sets from the parts of the radial part are useful to answer the questions, and those from the parallel part indicate the back-ground, far-field stress. Thus, the intrusions provided us with a rare opportunity to answer those questions.
To this end, we analyzed the orientation data of the Amakusa dike swarm by two approaches. First, theoretical stress trajectory pattern predicted by the 2D elastic model of Ode (1957) was fitted to the trends and spatial distribution of the dikes to determine far-field stress. Second, the 3D stress inversion technique of Yamaji and Sato (2011) was applied to the 3D dike orientations. The 3D data were partitioned into subsets according to the locations where they were obtained in the field.
As a result, we had consistent results from the two approaches. That is, the local σHmax orientations determined by the 3D stresses were roughly parallel to the stress trajectories inferred from the 2D elastic model. In addition, the technique can evaluate the frequency of highly overpressured magmas decreased with the distance from the central magma reservoir. The ratio of magma pressure to principal stress showed a negative correlation with distance from the central magma chamber. This result is consistent with the assumption of the 2D analysis.
The results of this study demonstrate the validity of the 3D stress inversion technique for radial and parallel dike swarms. The technique can determine 3D local stress conditions in parts of both radial and parallel dike swarms. The coincidence of 2D and 3D analyses indicate the validity of the assumptions of both the two analyses.