阿拉善地块主要断裂系统性质、阶段和构造背景研究

The Alxa Block is an important link not only between the main tectonic units in the western and eastern China, but also the transitional region between the Tethys Regime to the south and the Paleo Asian Regime to the north. Since the Paleozoic, the Alxa Block has undergone complicated evolution due to the frequent tectonic events occurring around the block. However, some key questions such as the range of the Alxa Block and its relationship between the North China Plate, the responses and deformation of the Alxa Block to the tectonic events and its function in these events are not well known. The application plans: (1) to work on the geometry, kinematics and ages of several important faults in the Alxa Block, and combined with data from different locations to figure out the stages, types and paleostress fields of different stages of these faults; based on these studies, to analyze the different paleostress fields of the Alxa Block and discuss the tectonic settings of the Alxa Block and its functions and responses to the different tectonic events; (2) to use SHRIMP II and/or LA-ICP-MS to date the Paleozoic diabase in the Hexi Corridor, to carry out the structural mapping of the northeastern region of the Alxa Block at large scale to figure out the outcrop pattern of the boundary between the Alxa Block and the North China Plate, to date the age of the boundary fault and compare the deformation and compositions across the boundary to fix the range the Alxa Block, the tectonic relation with the North China Plate and their amalgamation timing; (3) to carry out the structural mapping at large scale in key regions to fix the relationship between the Altyn Tagh fault and the Longshoushan Fault, and the Longshoushan Fault and the western fault of the North China Plate.

阿拉善地块是连接中国北方东西两侧主要构造单元的纽带，也是特提斯构和古亚洲洋构造域之间的过渡区。自古生代以来经历了复杂地质演化，但有关阿拉善地块的范围以及与华北板块的关系；古生代以来阿拉善对各重要构造事件的响应及所起的作用均不清楚。项目计划：(1) 开展阿拉善地块重要断裂几何学、运动学和年代学研究，并结合点、线观察确定这些断裂的活动性质、阶段和不同阶段的古应力场，在此基础上，综合分析阿拉善不同阶段古应力场，讨论各自构造背景以及阿拉善在重要构造事件中的作用和变形；(2) 利用离子或激光探针质谱技术，对河西走廊古生代辉绿岩进行锆石年龄测定，并在阿拉善东北缘开展大比例尺构造填图，分析阿拉善与华北板块边界的地表出露形式、活动年代以及两侧物质组成和变形表现，探讨阿拉善的范围以及与华北板块的作用方式和时代；(3)重点地区目标断裂大比例尺构造填图，确定阿尔金和龙首山断裂以及龙首山和华北西缘断裂之间的关系。

阿拉善地块作为中亚造山带的一部分，自晚古生代以来经历了从板块边缘到板内环境的多次构造事件。阿拉善地块的演化反映了周边板块边界所施加的应力平衡。本研究表明古亚洲洋闭合及后续阶段由地块拼合产生的挤压应力，形成了阿拉善地块与华北克拉通和小地块（379-351 Ma）之间的韧性剪切，该事件被解释为华北克拉通与阿拉善地块的拼合或这些地块之间的板块会聚引起。阿拉善地块发育几条区域性的晚二叠世东西向右行剪切带，它们是龙首山、巴丹吉林、北大山和雅布赖剪切带，它们可能是整个中亚造山带巨型剪切带的一部分，约3000公里长。阿拉善地块韧性变形的驱动力可能来自伊利地块向东的挤压。华北克拉通与扬子克拉通碰撞有关的远场效应，在阿拉善地块东部形成了一个大的左旋剪切带（250-210 Ma），西部的星星峡剪切带也为同时代的左旋剪切带。阿拉善块体在大范围的左旋剪切作用下发生变形，并逆时针旋转约30°。蒙古鄂霍次克海北段远场侏罗纪挤压板块终场闭合，古太平洋在欧亚板块之下向东低角度向西俯冲，拉萨-羌塘碰撞向南组合，使阿拉善地块从不同方向变形，形成具有不同走向的缩短构造，反转以前的伸展盆地。与长期俯冲有关的晚期拉张力和与古太平洋板块向东东退有关的晚期弧后伸展，导致阿拉善地块早白垩世玄武岩和同时代裂谷盆地的出现，并伴有各种伸展断层。晚白垩世，沿欧亚板块东南缘发生斜碰撞，新特提斯板块沿南部边界俯冲，导致沿阿拉善地块东部发育的左旋压剪作用，阿拉善地块内部的东西向伸展盆地反转。阿拉善地块东缘近南北走向的构造可能是一个长期存在的壳强度边界，目前这一边界形成了银川－河套地堑。青藏高原隆起区的重力势能是造成新生代阿拉善地块变形的主要原因。实地调查表明，阿尔金断裂带向东延伸，绕过了宽滩山与东部的黑山北部断裂相连，但不与阿拉善地块内部断裂相连。本研究在阿拉善地块内部发现一条活动左旋走滑断层（阿拉善走滑系），该走滑系的形成是青藏高原向北挤压的结果。