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

Small-scale ultramylonite and cataclasite bands, millimeters to tens of cen timeters thick, are developed in granitic rocks west of the Hatagawa Fault Zone (HFZ) in the Abukuma Belt, northeast Japan. They occur as single or pa ired bands with sharp planar boundaries trending NNE-SSW, and often form ne tworks and conjugate sets. The very small S-C angle and the high displaceme nt/thickness ratio of the bands suggest that the shear strain is high. The ultramylonite bands are commonly associated with cataclasite bands and mine ral veins, and rarely with pseudotachylyte. Some cataclasite bands contain mylonitized layers in which quartz fragments are strongly deformed and dyna mically recrystallized. On the other hand, some ultramylonites are fracture d 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 wit hout significant mass transfer. Mineralogy and microstructures of some ultr amylonites suggest the strong possibility that they are derived from pseudo tachylyte. The ultramylonite bands are interpreted as forming in the 10-15- km-deep cataclastic-plastic transition zone under greenschist facies condit ions where co-seismic fracturing and aseismic plastic flow have alternated. (C) 2000 Elsevier Science Ltd. All rights reserved.