PLASTIC FLOW OF ULTRABASIC ROCK IN SOUTHERN TIBET
藏南超基性岩的塑性流变

A study of plastic flow in some ultrabasic massifs located at the bottom of the Yarlung Zangbo ophiolite suite shows that these massifs have generally undergone plastic flow deformation. According to grain shape of mineral, main deformation phenomena, recrystallization degree and fabric charicteris-tics, the main plastic flow textures can be divided into the following three types: proto-texture-protogranular and protoplaty (mildly deformed facies), blastic texture-granular blastic and platy-blastic (moderately deformed facies), porphyroclastic mylonitic texture-porphyroclastic, mylonitic,and ultramylonitic (strong deformed facies). The first and the third are the main textural type of high temperatural plastic flpw, with the slip system being T (slip plane) = (001), t(slip direction) = 100] and T=(010), t = 100] respectively. The blastic texture is the main textural type of lower temperature plastic flow, with T=010], 100], and T=.001], 010].The flow stress value estimated based on dislocation density, subgrain and recrystallized grain size of olivine is 289× 105-5414 × 105Pa, increasing with intensifying strain.Compared with the slip system in olivine and flow stress value with the focal distribution of the Himalayan are vate seismic belt in the profile, it has been inferred that the bedded structure of the lithosphere formed by the crystal preferred orientation of olivine is one of the important reasons for discontinuous seismic wave velocity in the Himalayan area. Plastic flow in rocks is probably a major mode of releasing deep seismic energy and causing seismic activity.