帕米尔北缘木什背斜第四纪滑脱褶皱作用与北翼逆断裂的生长

QUATERNARY DETACHMENT FOLDING AND PROPAGATION OF NORTH LIMB FAULT OF MUSHI ANTICLINE,NORTHERN MARGIN OF THE PAMIR

发育在帕米尔弧形推覆构造带最前缘的木什活动背斜是一南缓北陡的第四纪滑脱褶皱,背斜的最小地壳缩短量为0.7km,构造隆升幅度可达1.5km。木什背斜北翼逆断层由一系列坡向北的反向断层陡坎组成,不同断坎间垂直位移分布呈现此消彼长的特征,不论是整个北翼逆断层西段还是单条断坎,其垂直位移均呈东高西低的不对称分布,位移梯度东高西低,这可能反映了背斜西段晚第四纪褶皱作用的东强西弱。该断层可能是背斜生长过程中为了调节褶皱逐渐紧闭时不断减小的核部空间形成的浅部、无根的前翼次级剪切逆冲断层,本身不会产生中强地震。该断层晚第四纪平均水平缩短速率为0.8mm/a,仅吸收了该区现今GPS地壳缩短速率的1/5。木什背斜和背斜北翼逆断层的生长均遵从根据全球断层数据获得的断层长度>100m的标度律。木什背斜北翼逆断层西段陡坎的幂律回归标度指数n=1.37(R2=0.88),其断层最大位移(Dmax)与断层长度(L)的比值(k)远小于木什背斜的约4.3%,但比全球地震地表破裂带的k值(10-4～10-5)要大1～2个数量级,这表明木什背斜北翼断裂陡坎可能是数个中强震的派生产物,但尚处于断层发育的早期。

The Mushi anticline locates at the frontier Pamir arcuate nappe tectonics belt（PFT）,which is a detachment fold with a gentle south limb and steep north limb,and its earth crustminimum shortening is ～0.7km with uplift up to 1.5km.The north limb fault of Mushi anticline is composed of a series of obsequent slope fault scarps,and the distribution of vertical displacements among different fault scarps presents a pattern of one increasing and the other decreasing.No matter of the entire western segment of the northern limb faults or a single fault,the displacement distribution is asymmetric,that is,high in the east and low in the west,and the same to displacement gradient.This may reflect the late Quaternary folding of Mushi anticline as being intensive in the east and feeble in the west.The fault may be a shallow,rootless secondary fault formed during the growth process of the anticline in order to accommodate the constantly decreased space of anticline nucleus as the fold tightened gradually.The late Quaternary shortening rate of the fault is 0.8mm/a,absorbing only one fifth of the nowadays crustal shortening rate of the region.The growth of Mushi anticline and the north limb fault of Mushi anticline both are in accordance with global fault dataset scaling relationship,that is,fault length is over 100m.The power-law regression scaling exponent of west segment of the northern limb fault of Mushi anticline is n=1.37（R2=0.88）,and its specific value（k）of maximum fault displacement and fault length is far less than that of the Mushi anticline,which is ～4.3%,but 1～2 orders of magnitude larger than that of global fault dataset（10-4～10-5）.This may show that the northern limb fault of Mushi anticline is the offshoot of several moderate strong earthquakes,and it is still in initial stages.