三角剪切断层传播褶皱在祁连山北缘断裂中的应用：以黑河口断层为例

APPLICATION OF TRISHEAR FAULT-PROPAGATION FOLDING TO THE NORTH FRONTAL THRUST OF THE QILIAN SHAN MOUNTAINS： AN EXAMPLE FROM THE HEIHEKOU FAULT

逆冲断层运动学过程研究可为造山作用和盆地演化提供重要的信息。位于青藏高原东北缘的祁连山北缘逆冲断裂（NFT）是一条主要的边界断裂，控制着祁连山晚新生代以来的隆升变形，但对于该断裂系统的运动学过程缺乏研究。甘肃张掖西南莺落峡附近，黑河口断层（祁连山北缘断裂的一部分）穿过红沙沟并导致其阶地面错断和褶皱变形。为了研究该区域褶皱变形特征和断层运动学过程，我们首先在红沙沟区域进行了详细的野外考察并对变形的地层和阶地面进行了测量，然后通过数值模拟方法来验证褶皱模型。红沙沟保存有9级河流阶地，利用差分GPS对阶地面的高程进行了测量，同时在野外测量了部分地层的产状。多处断层剖面表明该区域变形属于基底卷入的构造类型，另外变形的阶地面在近断层处表现出明显的褶皱形态。结合阶地面变形特征以及地层的变形，三角剪切断层传播褶皱模型被初步用于该区域构造变形的解释。模型模拟得到自T5、T6和T7形成以来断层滑动量分别是 44±7m、59±10m和 164±28m；结合T6年代得到自约9万年以来该断层的平均滑动速率为 0.68±0.19mm/a，垂直分量为 0.58±0.13mm/a。根据地层与阶地变形特征，并结合模型模拟结果，我们认为祁连山山前基底断层卷入的构造中，在断层出露地表之前，断层顶端存在一个分散式的三角形变形区，区域的应力状态控制着下伏地层和上覆地貌面的褶皱变形。

Thrusts kinematics can provide significance information for the orogen and basin evolution. Along NE margin of the Tibetan Plateau, the north frontal thrusts of the Qilian Shan（NFT）are main boundary faults controlling the uplift of the Qilian Shan Mountains in Late Cenozoic, while the kinematics of this fault system are poorly known. Near the Yingluo Gorge, southwest of Zhangye City, Gansu Province, the Heihekou Fault（one segment of the NFT）crosses the Hongshagou River and induces faulting and folding deformation of fluvial terraces along the river. The study area locates at the mountain pass of the Heihe River 30km southwest of Zhangye City. To study the folding characteristics and kinematics, we firstly conducted detailed field investigations and measurement on the deformed strata and terrace surfaces, and then carried on simulating work to test the fold model. Along the Hongshagou River, nine individual levels of fluvial terrace are preserved. Fluvial terrace heights were measured by the differential GPS method and the strata attitudes were measured in the field. Lithology sections crossing the fault suggest that the deformation in this region belongs to a basement-involved structure, and the deformed terraces show a clear folding area near the fault. Based on the deforming characteristics of terrace surfaces and the strata deformation, the trishear fault-propagation fold model is preliminarily applied in this region. Modelling results suggest that the fault slipped in the amounts of 44±7m, 59±10m and 164±28m respectively after the formation of T5, T6 and T7. Combined with the exposure age of T6 surface, we derived a slip rate of 0.68±0.19mm/a for the Heheikou thrust and a vertical component of 0.58±0.13mm/a since ca.90ka B.P. According to the deformation features of strata and terraces, and to the result of the model simulation on the basement-involved structures in the Qilian foreland, we can conclude that there is a triangular zone of deformation with distributed strain on the top of fault before the fault breaking out to the surface. The stress state of this region controls the folding geometries of strata and overlying geomorphic surfaces.