Tomoaki Tomita

Development of the Hatagawa Fault Zone clarified by geological and geochronological studies

The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.

Water-rock interaction observed in the brittle-plastic transition zone

Rock alteration and geochemistry of the fault rocks are examined to infer the characteristics of the fluid phase related to the ancient fault activity. The Hatagawa Fault Zone, northeast Japan, is an exhumed seismogenic zone which is characterized by close association of brittlely and plastically deformed fault rocks mostly derived from Cretaceous granitoids. Epidote and chlorite are dominant alteration minerals in both rocks. However, calcite is characteristically developed in the cataclastic part only. Decrease in oxygen isotope ratio and existence of epidote and chlorite, even in weakly deformed granodiorite, is evidence of water-rock interaction. The water/rock ratio is interpreted to be relatively small and fluid chemistry is buffered by host rock chemistry in the mylonite. The occurrence of calcite in brittle structures is explained by changes in water chemistry during shear zone evolution. CO2-rich fluid was probably introduced during cataclastic deformation and increased CO2 concentration resulted in precipitation of calcite.

Geochronological constraint on the brittle-plastic deformation along the Hatagawa Fault Zone, NE Japan

K-Ar ages and fission-track ages of granitic rocks in the Hatagawa Fault Zone (HFZ), NE Japan were measured to examine the cooling history of the HFZ. The HFZ is an NNW-SSE trending fault zone in the Cretaceous granitic rocks, and consists of a conspicuous cataclasite and two types of mylonite with a sinistral sense of shear. The cataclasite zone is NNW-SSE trending and continuous over at least 40 km with a maximum thickness of 100 m. One type of mylonite is low-T mylonite, which is mainly developed for a length of 6 km along the HFZ. The other is high-T mylonite, which is widely distributed in the HFZ. Most of K-Ar ages of hornblende and biotite from granitic rocks are about 110 Ma and show no obvious differences along the strike of the HFZ or among different granite bodies. This implies that the granitic rocks in the HFZ have a similar cooling history and cooled rapidly from closure temperature of hornblende to that of biotite. Zircon fission-track analysis shows little possibility of reheating of the granitic rocks. This supports the formation of cataclasite and mylonite during the cooling of the granitic bodies. Fission-track ages of zircon and apatite from the samples in and near the areas where the low-T mylonite is developed are older than those for other areas. Infiltration of near-surface derived water into the low-T mylonite after plastic deformation may account for the accelerate cooling of granitic bodies.

Growth of plastic shear zone and its duration inferred from theoretical consideration and observation of an ancient shear zone in the granitic crust

A new model for growth of plastic shear zone is proposed based on the basis of a theory of fluid dynamics coupled with a rheological constitutive function, and is applied to a natural shear zone. Mylonite, ultramylonite and other ductile fault rocks are well known to deform in a plastic flow regime. The rheological behavior of these kinds of rocks has been well documented as a non-linear viscous body, which is empirically described as , where : strain rate, τ: shear stress, Q: activation energy, R: universal gas constant, T: absolute temperature, and A and n are constants. Strain rate- and temperature-dependent viscosity is obtained by differentiating the equation, and simplified by substituting n = 1. Then, substitution of the equation into a diffusion equation, , derives an equation δ = 4[t/p · A exp(−Q/RT)]1/2, where δ: thickness of active layer of viscous deformation, ν: kinematic viscosity, and ρ: density. The duration of creep deformation along the ancient plastic shear zone (thickness: 0.076 m) is estimated to be around 760 s, in a temperature range from 300 to 500°C. This estimation is rather good agreement with intermittent creep during inter-seismic period, than steady state creep or co-seismic slip.