Kyuichi Kanagawa

Transformation-induced strain localization in a lherzolite mylonite from the Hidaka metamorphic belt of central Hokkaido, Japan

Abstract The Uenzaru peridotite complex in the northern part of the Hidaka metamorphic belt of central Hokkaido, Japan, contains a mylonitized, plagioclase lherzolite. The plagioclase-lherzolite mylonite consists of porphyroclasts of olivine, orthopyroxene, clinopyroxene and spinel, and fine-grained matrix of olivine, orthopyroxene, clinopyroxene, plagioclase and spinel. Symplectite composed of plagioclase, olivine and chromian spinel occurs around porphyroclasts of orthopyroxene, clinopyroxene and spinel. A fine-grained aggregate of plagioclase, olivine and chromian spinel also occupies the pressure shadows around porphyroclasts of orthopyroxene, clinopyroxene and spinel. The fine-grained aggregates in the pressure shadows laterally connect with each other to form layers, which characterize the mylonitic foliation. Symplectite with the same mineral assemblage as the fine-grained aggregate, a bulk rock chemistry of the mylonite close to that of pyrolite, and a reverse zoning of plagioclase such that the anorthite component increases toward its rim, indicate that the fine-grained aggregate is derived from the subsolidus phase-transformation reaction from spinel lherzolite to plagioclase lherzolite, but not from melt. The mylonitic foliation defined by layers of fine-grained aggregate implies that strain is localized into the reaction products. Therefore the phase-transformation reaction may have enhanced mylonitization of the lherzolite. Compositional zoning of pyroxenes and plagioclase in the mylonite suggests an adiabatic decompression at temperatures above 960°C within the spinel-lherzolite stability field, followed by a rapid isobaric cooling down below 760°C at about 700–800 MPa. The lherzolite which initially ascended as a mantle diapir was mylonitized through the phase-transformation reaction and thrusting onto crustal rocks, the contact with which resulted in a rapid cooling of the lherzolite.

Effects of dissolution-precipitation processes on the strength and mechanical behavior of quartz gouge at high-temperature hydrothermal conditions

Laboratory experiments on simulated quartz gouges at a temperature of 927°C, a confining pressure of 300 MPa, and a pore water pressure of 200 MPa demonstrate the interactions between cataclastic and dissolution-precipitation processes and their effects on the strength and mechanical behavior. Significant porosity reduction and microstructures, including grain interlocking, grain interpenetration, and widespread growth of euhedral-shaped grains, indicate the operation of dissolution-precipitation processes. Under conditions which favor the activity of dissolution-precipitation processes, i.e., for small grain size or at slow displacement rates, deformation is distributed across the whole gouge layer, and strength increases continuously with increasing displacement, so that coefficients of friction reach to 0.7. In contrast, for specimens with large grain size sheared at fast displacement rates, deformation is localized along a gouge-forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi-stable sliding with coefficients of friction as low as 0.45. Analysis of our results indicates that where the kinetics of dissolution-precipitation is sufficiently fast to partly accommodate shear strain, the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening; in contrast, where cataclastic processes predominantly accommodate shear strain, the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening.

Striped iron zoning of olivine induced by dislocation creep in deformed peridotites.

Deformation of solid materials affects not only their microstructures, but also their microchemistries. Although chemical unmixing of initially homogeneous multicomponent solids is known to occur during deformation by diffusion creep, there has been no report on their chemical zoning due to deformation by dislocation creep, in either natural samples or laboratory experiments. Here we report striped iron zoning of olivine ((Mg,Fe)2SiO4) in deformed peridotites, where the iron concentration increases at subgrain boundaries composed of edge dislocations. We infer that this zoning is probably formed by alignment of edge dislocations dragging a so-called Cottrell ‘atmosphere’ of solute atoms (iron in this case) into subgrain boundaries during deformation of the olivine by dislocation creep. We have found that the iron zoning does not develop in laboratory experiments of high strain rates where dislocations move too fast to drag the Cottrell atmosphere. This phenomenon might have important implications for the generation of deep-focus earthquakes, as transformation of olivine to high-pressure phases preferentially occurs in high-iron regions, and therefore along subgrain boundaries which would be preferentially aligned in plastically deformed mantle peridotites.

Simulated pressure fringes, vorticity, and progressive deformation

Abstract An alternative approach to numerically simulating displacement controlled rigid fiber growth in pressure fringes takes account of rigid-body rotation of an object and its surrounding pressure fringe according to their shape and flow in the matrix. Provided that the process of fiber growth is known, both syntaxial and antitaxial rigid fiber growths in pressure fringes can be numerically simulated by this method for any plane, isochoric, Newtonian viscous flow with constant flow parameters. Three syntaxial and two antitaxial growth models are suggested, but it is uncertain yet which model is appropriate for natural fiber growth in pressure fringes. The method is illustrated for the rigid fiber models of syntaxial and antitaxial fiber growths around a circular rigid object. The lengths and curvatures of fibers in a simulated pressure fringe are constant except for a few fibers in these models and they are shown to vary systematically according to finite strain and kinematic vorticity number. Although the growth process of pressure fringes is required to be known, simulated fibers provide a powerful tool for quantitative estimates of vorticity and progressive deformation.

Frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution

Two recent experimental studies on the frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution are compared. One is triaxial shear experiments on quartz gouge at high pressure-temperature hydrothermal conditions (Kanagawa et al., 2000), and the other is rotary shear experiments on halite gouge at atmospheric pressure and room temperature in the presence of methanol-water mixtures (Bos et al., 2000). In spite of quite different experimental settings and conditions, the results of these two series of experiments are strikingly similar; both cataclasis and pressure solution being active during the experiments, gouge strength rate-controlled by cataclasis, two different frictional behaviors of slip hardening and softening, slip hardening associated with gouge compaction, distributed deformation and wall-rock failure, slip softening associated with localized slip along the gouge-wall-rock interface, and the transition from slip-hardening to slip-softening behavior according to decreasing rate of pressure solution. Although there is a difference in velocity dependence of strength between quartz and halite gouges, these similarities clearly demonstrate the important effects of pressure solution on the frictional behavior of gouge-bearing faults.

Frictional strength of ground dolerite gouge at a wide range of slip rates

We conducted a series of rotary-shear friction experiments on ground dolerite gouges, in which the amount of adsorbed moisture increases with grinding time (tgr), at room temperature and humidity, a normal stress of 2 MPa, and constant equivalent slip rates (Veqs) ranging from 20 µm/s to 1.3 m/s. Their frictional strength changed with Veq and tgr in three different ways depending on Veq and the gouge temperature (T). At Veq ≤ 1.3 cm/s, T did not exceed 80°C, and the steady state friction coefficient (μss) ranged from 0.59 to 0.80. μss changes little with Veq, while μss at a given Veq systematically increases with tgr probably due to moisture-adsorbed strengthening of gouges. At Veq = 4 cm/s, T exceeded 100°C, and dehydration of gouges resulted in roughly the same μss values (0.60–0.66) among gouges with different periods of tgr. At Veq ≥ 13 cm/s, T reached 160–500°C, and μss dramatically decreases with Veq to 0.08–0.26 at Veq = 1.3 m/s, while μss at a given Veq systematically decreases with tgr. At these fast Veqs, dehydration of gouges likely occurred too fast for water vapor to completely escape out from the gouge layer. Therefore, faster dehydration at faster Veq possibly resulted in a larger pore pressure increase and lower frictional strength. In addition, because gouges with longer periods of tgr contain larger amounts of adsorbed moisture, they became weaker due to larger increases in pore pressure and hence larger amounts of reduction in frictional strength.

A structural traverse across the Shimanto belt in western Shikoku, Japan

The Cretaceous and Tertiary Shimanto accretionary complex is largely characterized by imbricated thrust slices of trench-fill and ocean-floor sediments, and is thought as an ancient analog of the Nankai accretionary prism. Recent studies on a thermal structure and fault rock analysis for the Shimanto accretionary complex in the central and eastern Shikoku revealed that it has suffered earthquake faulting along the out-of-sequence thrusts associated with tectonic uplift. However, special distributions of thermal and tectonic structures are remaining unclear since those in the western part of Shikoku are poorly understood. In the presentation, we demonstrate the distributions and details of deformed rocks (e.g. melange and brittle faults), geological structure, and vitrinite reflectance across the Shimanto belt in western Shikoku.