Ultramylonite bands derived from cataclasite and pseudotachylyte in granites, northeast Japan

Small-scale ultramylonite and cataclasite bands, millimeters to tens of centimeters thick, are developed in granitic rocks west of the Hatagawa Fault Zone (HFZ) in the Abukuma Belt, northeast Japan. They occur as single or paired bands with sharp planar boundaries trending NNE-SSW, and often form networks and conjugate sets. The very small S-C angle and the high displacement/thickness ratio of the bands suggest that the shear strain is high. The ultramylonite bands are commonly associated with cataclasite bands and mineral veins, and rarely with pseudotachylyte. Some cataclasite bands contain mylonitized layers in which quartz fragments are strongly deformed and dynamically recrystallized. On the other hand, some ultramylonites are fractured producing fragments that have rotated during later cataclasis. The major element content of the ultramylonite bands is similar to that of the surrounding granitic rocks, strongly suggesting that the ultramylonite bands have formed through in-situ deformation of the granitic protolith without significant mass transfer. Mineralogy and microstructures of some ultramylonites suggest the strong possibility that they are derived from pseudotachylyte. The ultramylonite bands are interpreted as forming in the 10-15-km-deep cataclastic-plastic transition zone under greenschist facies conditions where co-seismic fracturing and aseismic plastic flow have alternated. (C) 2000 Elsevier Science Ltd. All rights reserved.