南北地震带中段地貌发育差异性及其与西秦岭构造带关系初探

DIFFERENTIAL LANDSCAPE DEVELOPMENT OF THE CENTRAL N-S SEISMIC ZONE AND ITS RELATION TO THE WEST QINLING TECTONIC BELT

南北地震带中段地区发育一系列与西秦岭构造带走向近似平行的断裂系统,历史上沿上述断裂系统曾发生过多次强震甚至大地震,预示着断裂系统的最新活动特征,同时也表征了南北地震带中段地区断裂活动的差异性。文章选取南北地震带中段西秦岭构造带交接区长江水系嘉陵江一级支流白龙江和西汉水,黄河水系一级支流洮河以及渭河等上游流域为研究对象,以宏观地貌发育、水系平面展布和河流纵剖面陡峭程度等特征为基础数据,初步分析了南北地震带中段地貌发育总体特征及其与西秦岭构造带的关系。研究发现,地貌特征指示南北地震带中段明显存在构造活动性差异特征:南侧白龙江流域表现为地形起伏大、坡度陡、河流陡峭指数高等特征,主要受控于迭部-白龙江断裂带的持续活动;北侧西秦岭北缘断裂带发育部位,武山以西断裂迹象明显,渭河流域上游与洮河岷县-临洮南段等地区的地貌参数也指示了该段断裂较强的活动性。渭河及嘉陵江上游西汉水地区水系分布不对称特征指示了晚新生代以来西秦岭造山带与渭河地堑系活动的共同控制。白龙江水系主要支流所表现出的平行展布的线性水系特征表明迭部-白龙江断裂带等一系列平行断裂带对水系发育的控制作用。结合以往研究成果、历史强震和大震以及微震分布资料,我们进一步分析发现,南北地震带中段存在一条地震密集带,该带沿岷山-龙门山构造带向北,至舟曲-迭部附近跨过白龙江,向北沿礼县、宕昌及岷县至天水-甘谷-武山一带再向北经通渭、西吉等地区与六盘山构造带相接。这一认识支持南北向构造特征晚于东西向构造体系,并且很大程度上是深部地幔动力学和岩石圈圈层关系调整变动的产物的认识,同时也揭示了南北地震带与西秦岭构造带复杂的叠加复合构造关系。

Around the central N-S Seismic Zone,there are a series of strike-parallel fault system along the West Qinling Tectonic Belt. Several strong earthquakes occurred in the history on those faults, indicating the latest and differential fault activities around this region. In this paper,we will target upstream areas of the Bailongjiang River, Xihanshui River （two first order tributaries of Jialingjiang River of Yangtze River Basin） , and Taohe River,Weihe River （two first order tributaries of the Yellow River Basin）, and further analyze macro-scale landscape features, planar distribution of drainage networks,and river channel steepness index. Based on all of these geomorphometric parameters,we will discuss the general landscape characters around the central N-S Seismic Zone. We finally find that the landscape development is indicative to the differential tectonic acitivities. The highest local relief,slope and channel steepness along the Bailongjiang River are resulted from continued movement along the Deibu-Bailongjiang Fault. We also observe active features, indicated by higher geomorphometric parameters, where the fault trace is obvious west of Wushan along the West Qinling Fault, and near upstream area of the Weihe River and between Minxian city and south of Lintao city along the Taohe River. Asymmetric drainage pattern of the Weihe River and Xihanshui River might have been resulted combined control from activities of West Qinling and Weihe Graben in Late Cenozoic. Meanwhile, the parallel distribution of the linear drainage feature of the Bailongjiang River is representative of the control of parallel distribution of the Deibu-Bailongjiang fault and other faults. By analyzing the previous result and earthquake distribution, we further notice that there is an earthquake concentration swath, which is along the Minshan-Longmenshan Tectonic Belt, northward across the Bailongjiang River between Zhouqu and Diebu cities, and further northward around Lixian, Tanchang, Minxian cities, and then near Tianshui-Gangu- Wushan, and Tongwei, Xiji, finally joining the Liupan Shan Tectonic Belt. Our finding support the previous knowledge that the N-S trending tectonic feature is younger than W-E system and developed by deep mantle dynamics and adjustment of different layers of the lithosphere. In addition, our present study presents the complex structural relationship of central N-S Seismic Zone and West Qinling Tectonic Belt.