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[SCG55-08] Constraints on driving fluid pressure ratios from dilation directions of dikes
Keywords:dike, mineral vein, fluid pressure, stress tensor inversion
Dilatant fractures such as igneous dikes and mineral veins are clues to states of crustal stress and fluid pressure. Recent stress tensor inversion techniques to analyze orientation distributions of a set of dilatant fractures provide reduced stress tensors which carry principal stress axes and stress ratios (shape parameter of stress ellipsoid). Furthermore, a representative value of the driving fluid pressure ratio, which is the ratio of difference between fluid pressure and sigma_3 (minimum compressional principal stress) to the differential stress, can be estimated by the techniques. However, individual values of fluid pressures acting on fractures has not been investigated. This study proposes a method to estimate individual driving fluid pressure ratios on fractures by using dilation directions of fractures.
When a dilatant fracture wall has a planar shape, it is difficult to determine its dilation direction on outcrops. If the wall has a shape of polygon or curved surface, we can identify or constrain the dilation direction by fitting walls of both sides. The dilation direction is expected to reflect the ratio between normal and shear stresses exerted on the fracture when it dilates. Since the normal stress is diminished by the pore fluid pressure along the fracture, the dilation direction provides information on fluid pressure.
Firstly, this study performed theoretical calculations to assess the sensitivity of dilation directions to the driving fluid pressure ratios. As a result, enough sensitivity was confirmed for various orientations of dilatant fractures except for those nearly perpendicular to sigma_1 (maximum compressional principal stress) axes. Then a new inversion method was developed to determine driving fluid pressure ratios by searching for optimal value to fit the calculated traction vectors to the observed dilation directions.
The method was applied to the early Miocene andesitic dike swarm intruded into Mino-Tamba Belt around Tsuruga bay, Fukui Prefecture. The reduced stress was calculated by the conventional method of stress tensor inversion (Sato et al., 2013). The calculated values of driving fluid pressure ratios range from 0 to 0.8. A clear correspondence between the driving fluid pressure ratios and normal stresses except for fluid pressures is found. This fact suggests a static formation of dikes, i.e., dikes formed when the magma pressure slightly exceeds the normal stress.
Reference
Sato, K., Yamaji, A. and Tonai, S., 2013, Tectonophysics, 588, 69-81.
When a dilatant fracture wall has a planar shape, it is difficult to determine its dilation direction on outcrops. If the wall has a shape of polygon or curved surface, we can identify or constrain the dilation direction by fitting walls of both sides. The dilation direction is expected to reflect the ratio between normal and shear stresses exerted on the fracture when it dilates. Since the normal stress is diminished by the pore fluid pressure along the fracture, the dilation direction provides information on fluid pressure.
Firstly, this study performed theoretical calculations to assess the sensitivity of dilation directions to the driving fluid pressure ratios. As a result, enough sensitivity was confirmed for various orientations of dilatant fractures except for those nearly perpendicular to sigma_1 (maximum compressional principal stress) axes. Then a new inversion method was developed to determine driving fluid pressure ratios by searching for optimal value to fit the calculated traction vectors to the observed dilation directions.
The method was applied to the early Miocene andesitic dike swarm intruded into Mino-Tamba Belt around Tsuruga bay, Fukui Prefecture. The reduced stress was calculated by the conventional method of stress tensor inversion (Sato et al., 2013). The calculated values of driving fluid pressure ratios range from 0 to 0.8. A clear correspondence between the driving fluid pressure ratios and normal stresses except for fluid pressures is found. This fact suggests a static formation of dikes, i.e., dikes formed when the magma pressure slightly exceeds the normal stress.
Reference
Sato, K., Yamaji, A. and Tonai, S., 2013, Tectonophysics, 588, 69-81.