11:00 AM - 11:15 AM
[SMP25-08] Rheological Properties of Ductile Shear Zone Developed in Mado Megamullion, Shikoku Basin, Philippine Sea
Keywords:Oceanic Core Complex, Ductile shear zone, Back-arc basin, lower oceanic crust, gabbro, flow law
Oceanic core complexes (OCCs) are domal bathymetric features that are found along intermediate-to ultraslow-spreading ridge (<55 mm/yr) and are characterized by exposed lower crust and mantle material. These domal bathymetric features are interpreted to be exposed on the seafloor over several million years by detachment faulting, but few studies have examined their properties from a rheological point of view.
In this study, we investigate the kinematic features of detachment faulting in the Mado Megamullion, one of the OCCs in the Shikoku Basin of the Phillipine Sea, based on the deformation microstructure and crystallographic orientation fabric of the deformed gabbroic rocks.
Microstructural observations, crystallographic-preferred orientation (CPO) analysis and major element composition analysis of plagioclases and amphibole from 3 deformed gabbroic rocks (R10, R19a and R20) were studied among 10 rocks collected from the Mado Megamullion. The deformation mechanism was estimated based on the results of microstructural and CPO analyses. In addition, the deformation temperature was estimated by applying the chemical composition obtained from the major element composition analysis to the amphibole–plagioclase geothermometer.
Both plagioclase and the secondary amphibole show evidence of intracrystalline plastic deformation and were dynamically recrystallized. The CPOs of plagioclase and amphibole show a strong concentration of (001)[100] and (100)[001] patterns, respectively. The chemical compositions of plagioclase and amphibole are diverse, ranging from An41–An76 and Hornblende–Pargasite, respectively. However, the equilibrium temperatures obtained for 3 samples were in the same temperature range (870–690°C). Equilibrium temperatures of 690–610°C were obtained from felsic veins penetrating the gabbros.
Based on microstructural observations and CPO analyses, the deformation mechanism of plagioclase and amphibole is assumed to be dislocation creep. Assuming that the equilibrium temperature is the temperature range during deformation, the analyzed gabbros were plastically deformed at 870–690°C (granulite-facies) because the crystallographic slip planes of plagioclase and amphibole are the same.
In order to clarify the characteristics of the ductile shear zone, deformation mechanism maps of plagioclase deformed by dislocation creep were constructed from the dynamic recrystallized grain size, the equilibrium temperature and the flow law of plagioclase. The shear strain rate of the ductile shear zone of the Mado Megamullion was estimated to be more than 10–9 s–1 and its thickness to be about 20 cm. The values of shear strain rate and shear zone thickness estimated in this study mean that there are localized regions of high shear strain rate in the ductile shear zone. In other words, it suggests strain localization due to grain size reduction and fluid infiltration. We compared the deformation temperatures and shear strain rates obteined in this study with data from Godzilla Megamullion, the world's largest OCC in the Parece Vela Basin. The properties of the ductile shear zones of Mado Megamullion and Godzilla Megamullion show similarities. This suggests that there is a common feature in the formation process of Mado Megamullion and Godzilla Megamullion.
In this study, we investigate the kinematic features of detachment faulting in the Mado Megamullion, one of the OCCs in the Shikoku Basin of the Phillipine Sea, based on the deformation microstructure and crystallographic orientation fabric of the deformed gabbroic rocks.
Microstructural observations, crystallographic-preferred orientation (CPO) analysis and major element composition analysis of plagioclases and amphibole from 3 deformed gabbroic rocks (R10, R19a and R20) were studied among 10 rocks collected from the Mado Megamullion. The deformation mechanism was estimated based on the results of microstructural and CPO analyses. In addition, the deformation temperature was estimated by applying the chemical composition obtained from the major element composition analysis to the amphibole–plagioclase geothermometer.
Both plagioclase and the secondary amphibole show evidence of intracrystalline plastic deformation and were dynamically recrystallized. The CPOs of plagioclase and amphibole show a strong concentration of (001)[100] and (100)[001] patterns, respectively. The chemical compositions of plagioclase and amphibole are diverse, ranging from An41–An76 and Hornblende–Pargasite, respectively. However, the equilibrium temperatures obtained for 3 samples were in the same temperature range (870–690°C). Equilibrium temperatures of 690–610°C were obtained from felsic veins penetrating the gabbros.
Based on microstructural observations and CPO analyses, the deformation mechanism of plagioclase and amphibole is assumed to be dislocation creep. Assuming that the equilibrium temperature is the temperature range during deformation, the analyzed gabbros were plastically deformed at 870–690°C (granulite-facies) because the crystallographic slip planes of plagioclase and amphibole are the same.
In order to clarify the characteristics of the ductile shear zone, deformation mechanism maps of plagioclase deformed by dislocation creep were constructed from the dynamic recrystallized grain size, the equilibrium temperature and the flow law of plagioclase. The shear strain rate of the ductile shear zone of the Mado Megamullion was estimated to be more than 10–9 s–1 and its thickness to be about 20 cm. The values of shear strain rate and shear zone thickness estimated in this study mean that there are localized regions of high shear strain rate in the ductile shear zone. In other words, it suggests strain localization due to grain size reduction and fluid infiltration. We compared the deformation temperatures and shear strain rates obteined in this study with data from Godzilla Megamullion, the world's largest OCC in the Parece Vela Basin. The properties of the ductile shear zones of Mado Megamullion and Godzilla Megamullion show similarities. This suggests that there is a common feature in the formation process of Mado Megamullion and Godzilla Megamullion.