利用变形河流阶地限定帕米尔北缘木什背斜的缩短、隆升和侧向扩展

摘要

Locating at eastern end of the Pamir Front Thrust( PFT), the Mushi anticline grows initiating from early-Pleistocene till now. The anticline, with a gentle south limb and steep north limb, outcrops Pliocene Atushi formation and lower-Pleistocene Xiyu formation. Topographic profiles and drainage pattern indicate the lateral growth of the anticline from west to east. Combining mapping data and seismic profiles from the neighboring area, we find the Mushi anticline is a detachment fold, with a total shortening of ～0. 7kin and a total uplift up to ～ 1.5kin.Northern part of the anticline is dominated by a series of wide, flat terraces. According to OSL samples,the age of the terrace T2a,T3and T4 is 15.8±2. 4ka,55. 1±10. 3ka and 131.4±23.9ka respectively. Correlating with Marine Isotopic stages( MIS), the formation of terraces has some relationship with global climate change.As growing of the anticline, terrace surfaces deformed obviously, which is characterized by fault scarps, surface tilting or back-tilting, folding scarps and lateral tilting. Deforming patterns of the terrace surfaces indicate the Mushi anticline grows by limb rotation in late-Pleistocene. Using calculating models,we can confine the minimum shortening rate is 1.6±0. 3mm/a and the minimum uplift rate is 1.9±0.3mm/a.Longitudinal profiles of terraces indicate the Mushi anticline grows laterally through limb rotation. According to relationship between uplift and lateral propagation,we can acquire a faster eastward lateral propagation rate of the anticline during the period of 131 ～ 16ka,with a rate about 14. 6±3. 6mm/a; however,since 16ka,the rate reduced to 1.7±0. 3mm/a,implying the anticline tip stopped propagating to the east,and growing of the anticline was mainly dominated by lateral limb rotation in late Quaternary.%位于帕米尔前缘逆冲推覆体(Pamir Front Thrust,PFT)东端的木什滑脱背斜,是帕米尔弧形推覆构造带最前缘和最新的变形带.对地形横剖面、纵剖面和水系发育特征的分析表明,木什背斜总体上具有由西向东扩展生长的特征.在背斜核部及北翼发育数级开阔平坦的沿轴向展布的河流阶地,阶地可划分为4期.利用阶地堆积细颗粒石英光释光测年获得阶地面T2a、T3和T4的形成年龄分别为(15.8±2.40)ka、(55.1 ±10.3)ka、(131.4±23.9)ka.伴随背斜的生长扩展,河流阶地面发生了横向和纵向掀斜,并形成断层陡坎和褶皱陡坎.木什背斜晚第四纪的缩短和隆升主要是通过褶皱翼旋转机制进行的,估算其最小缩短速率为(1.6±0.3)mm/a,最小隆升速率为(1.9±0.3)mm/a.与此同时,沿轴向背斜发生了向东的侧向迁移和旋转.根据背斜垂直隆升与侧向扩展之间的关系,估算背斜在131～16ka期间向东的侧向迁移扩展速率较快,为(14.6±3.6)mm/a;自16ka至今,侧向迁移扩展速率迅速减小至(1.7±0.3)mm/a,背斜向东的迁移扩展可能已基本停止,而以侧向旋转为主.