Ikuo Hara

A new estimate of the conditions for transition from basal 〈a〉 to prism [c] slip in naturally deformed quartz

Abstract Quartz c -axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c ], prism 〈 a 〉 and basal 〈 a 〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c -axis fabric patterns change from girdle type, through X -maximum with a faint crossed girdle, to X -maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c -axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c ] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X -maximum c -axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a 〉 to prism [ c ] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.

c-Axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid-rich greenschist conditions

The quartz v-axis orientations and microstructures have been analyzed in an originally single-crystal buckled quartz vein of greenschist metamorphic grade from the Sambagawa metamorphic belt, southwest Japan. The basal (0001) plane was inclined by ca 10–20 to the quartz vein surface before deformation, and the flexure (essentially kink) which produced the fold was accomplished by basal (0001) slip alone in the hinges (i.e. flexural-slip folding). In addition to crystal plasticity, dissolution microstructures are ubiquitous in the quartz vein. Dynamic recrystallization resulted from both subgrain rotation and grain-boundary migration. Rotation recrystallization associated with basal (0001) slip resulted in host-controlled v-axis orientation distribution along a great circle, whereas migration recrystallization, perhaps assisted by intergranular fluid, resulted in a penetrative development of a weak X (elongation)-maximum v-axis fabric nowhere rotated by the folding. The migration recrystallization is evidenced by large v-axis misorientations across the grain boundaries, as well as irregular grain shape. Based on the weak v-axis fabric development and lack of grain shape fabric, solution-precipitation creep may have dominated in the fluid-assisted grain-boundary migration regime, and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein.

Relationship between growth of albite porphyroblasts and deformation in a sambagawa schist, Central Shikoku, Japan

Abstract Albite porphyroblasts in a basic schist of the Sambagawa metamorphic belt (Besshi—Bashi, Central Shikoku) show zonal variation of inclusions defined by inclusion-free mantles and inclusion-rich cores, though matrix minerals are partly incorporated in the outer zones of the mantles. From comparison of the crystallographic and dimensional fabrics of inclusion and matrix epidote, it has been concluded that the orientation of the principal axes of strain was different for the deformation of matrix fabrics and inclusion fabrics (fabrics produced before formation of cores). Inclusion amphibole in the cores belongs to the actinolite—common hornblende group, with Si content more than 7.0, whereas amphibole in the matrix and outer zones of mantles belongs to the common hornblende group, with Si content less than 7.1. This shows that the metamorphic temperature was higher during the phase of formation of the outer zones of the mantles than during that of the cores. The average direction of maximum growth of the mantles, which has been estimated from differences between the average shapes of porphyroblasts and those of cores, is parallel to the linear orientation of amphibole and epidote in the matrix. The average direction of minimum growth of the mantles coincides with the normal to the schistosity of the matrix. The mantles appear to be of the same generation as the matrix fabrics.

Boundary between subbasal I and subbasal II fields of quartz deformation in geological conditions

Abstract The Sangun crystalline schists of the Hirose district, Yamaguchi Prefecture, Southwest Japan, show progressive metamorphism from the pumpellyitechlorite facies, through the pumpellyite-actinolite facies, to the epidoteglaucophane-schist facies. The quartz fabrics produced during the main phase of metamorphism have been examined in this paper. Quartz microstructures in the low-temperature zone of the pumpellyite-chlorite facies are characterized by undulatory extinction and deformation lamellae which are referred to as type I subbasal lamellae, suggesting that the quartz deformation occurred in the subbasal I field. In the pumpellyite-actinolite facies, the epidote-glaucophane-schist facies and the high-temperature zone of the pumpellyite-chlorite facies, the quartz fabrics are characterized by small quartz grains produced by syntectonic recrystallization of coarsegrained quartz and crossed-girdle orientations of c-axes whose inclination angles are between 17° and 25°, suggesting that the quartz deformation occurred in the subbasal II field. The boundary between subbasal I and subbasal II fields of quartz deformation during the main phase of metamorphism appears to have been placed in the pumpellyite-chlorite facies. The temperature and confining pressure at the boundary are inferred to be at ca. 250 °C and 5–7 kb respectively.

Outcrop of shear zone along the Kawakami fault, the Median Tectonic Line active fault system at Yuyaguchi, Tambara Town, Ehime Pref., Western Shikoku

愛媛県丹原町湯谷口の中山川河床で, 中央構造線活断層系川上断層の大規模な露頭が現れた. 2001年梅雨時期の増水により, 河床堆積物が洗い流され, 断層露頭が露出したと考えられる. 露頭の広がりは長さ約110mで, 幅約20mに及ぶ. 露頭は市ノ川～砥部時階の中央構造線断層露頭(岸ほか, 1996)より約100m下流に位置しており(第1図, 第2図), 岡田(1972)が報告した川上断層露頭の東方延長にあたる. 断層の走向はN75°Eで, 傾斜はほぼ鉛直である. 鮮新統～中期更新統の岡村層相当層の礫層および砂層は, 断層近傍では地層はほぼ直立している. また, 礫層を構成する礫の長軸は断層に沿ってほぼ水平に配列している(第4図). 和泉層群の破砕帯の幅は約20mである. 岡村層相当層の礫層と接するところでは, 幅約1mの厚さで礫層と粘土化した和泉層群が混在している(第5図). その南側は約1mの幅で著しく粘土化しており, 黒色, 灰白色, 褐色, 灰緑色を呈する粘土化帯が縞状の構造を示している. 縞状構造はほぼ鉛直で, 断層面にほぼ平行である(第5図, 第6図). さらに, その南側は破砕岩片がやや多く含まれる粘土化帯よりなる.