川西北松潘-甘孜褶皱带较场弧形构造显微应变机制

The micro-strain mechanism of Jiaochang arc structure in Songpan-Garze fold belt,northwestern Sichuan, China

区域显微结构构造特征及其变形机制已经成为造山带运动学、动力学机制及模型的重要约束。对松潘-甘孜褶皱带较场弧形带15个定向构造样品显微薄片的研究表明:弧形带以中—浅层共轴挤压同变形-变质作用为主,形成残缕构造、对称眼球构造、膝折构造等;弧形带构造变形具有典型对称性应变的特征,东西两翼应变强度大致相当(γ≌1.8)、压缩率大致相等(C≌27%),由北向南相同单元内应变特征具有一定的增强趋势(而不同单元之间具有微弱减小的趋势),尤其是弗林指数K值(K>1)由北向南具有明显增大的特征,总体为三轴长椭球应变,为伸长型应变型式。较场弧形带的显微应变特征与典型弧形构造的"走滑成因"机制非对称性的特征迥异,其成因应归于以南北向主应力所控制的差异应力-应变机制。

The Jiaochang arc belt is characterized by synorogenic, multiphasic, coaxial compressional deformation and metamorphism, which is under the mid-shallow layer setting, and has some typical microtectonics, such as rotated helicitic structure, symmetric pres-sure shadow and kind structure etc. Based on 15 orientated samples′ microscopic features, dominant striking micro-strain feature of Jiaochang is symmetry, meanwhile magnitude of strain is roughly similar （γ≌1.8） in the western and eastern arc limbs, and percentage of compression is roughly equal （C≌27%） too. There is increasing tendency for micro-strain feature in the same unit from north to south, while there is decreasing tendency in different units, as well as the Flinn index （K1）. The strain pattern is triaxial prolate strain. The symmetrically micro-structural feature in Jiaochang is significantly different to the unsymmetrical feature about ＂strike-slip＂ mechanism of arc belt, attributed to differential stress-strain mechanism controlled by the north to south main stress.