Collaborative Research: Initiation and Long-Term Slip History of the Altyn Tagh Fault System, Northern Tibetan Plateau, NW China: A Tertiary Basin Piercing Point Study

阿尔金-塔格断裂系位于青藏高原的北缘，是印度和欧亚大陆碰撞形成的最大、最引人注目、也是最神秘的地貌之一。尽管对阿尔金断裂的构造地质和滑动历史的认识普遍较差，但它的构造意义一直是人们猜测的对象，该断裂在喜马拉雅-西藏造山带的模型中占有突出地位。近年来，为了更好地了解阿尔金-塔格断裂，人们主要使用间接方法来解释断裂的起始年龄和位移大小，或使用大地测量技术来解释滑动速率。利用地质穿透点直接研究断层位移还没有被广泛尝试。因此，关于阿尔金断裂的错动历史存在三个关键问题：(1)阿尔金断裂的起始时代是什么？(2)阿尔金断裂的滑动速率是如何随地质年代而变化的？(3)阿尔金断裂的滑动强度是如何沿阿尔金断裂长度变化的？为了回答这三个问题，必须在断层的每一段上识别出预滑和多个同滑穿刺点。这些穿刺点将允许确定断层的每一段(西、中、东)的错距历史，从而显示滑动幅度、滑动速率以及这些参数沿断层长度的变化。这项研究将通过识别阿尔金断裂截断的10个第三纪盆地的始新世-上新世沉积-源组合，直接研究阿尔金断裂的第三纪滑动历史和断层上滑动的分布。通过从单个盆地中识别不同时代的多个穿透点，可以在整个第三纪重建该段断层的滑动历史。此外，通过研究沿断层长度的一系列盆地，可以记录沿走向的位移变化。这些结果将提供一个稳健和明确的数据集，从中可以直接确定断层的时间和速度，这也将使关于沿着断层的滑动幅度的系统性变化的想法得到测试。PI以前在阿尔金断裂上的工作(Ritts，1998；Yue和Liou，1999；Ritts和Biffi，2000；Sobel等人，2001；Yue等人，在回顾中)已经证明了他们识别和记录阿尔金Tagh断裂上各种时代和各种类型的穿透点的能力。研究内容包括阿尔金断裂截断的10个第三系沉积盆地的岩石地层学和年代地层学、物理沉积学和岩相、物源和古水流等方面的文献。两个主要的野外季节将集中于沉积盆地的特征以及断层对面潜在的沉积物来源地体。第三纪盆地的野外工作将包括测量剖面的描述，古水流和砾岩成分数据的收集，以及地质填图。潜在沉积物来源地区的野外工作将侧重于岩性描述和中国制图的验证，以及收集潜在来源单位以进行分析工作。分析技术将应用于砾岩碎屑岩和潜在的物源单元，以检验基于野外数据提出的沉积物-物源对比。这些技术将包括岩相学检查、痕量和常量元素地球化学，以及铀-铅和40Ar/39Ar地质年代学。将利用泥岩的生物地层学分析来确定第三系单元的年龄。这项研究将约束第三系盆地的地质，以建立一系列的时间-位移曲线来表征阿尔金断裂的历史和沿走向的变化。由此得到的数据将更好地确定阿尔金断裂在容纳印度和亚洲之间的新生代会聚中所起的作用，从而区分挤压构造等机制在大陆变形中的相对重要性。

The Altyn Tagh fault system, which delineates the northern edge of the Tibetan Plateau, is one of the largest and most striking, yet also most enigmatic features attributed to collision between India and Eurasia. In spite of a generally poor understanding of the structural geology and slip history of the Altyn Tagh fault, its tectonic significance has been the subject of much speculation, and the fault figures prominently in models of the Himalayan-Tibetan orogen. Recent attempts to gain better knowledge about the Altyn Tagh fault have primarily used indirect methods to interpret age of fault initiation and magnitude of displacement, or geodetic techniques to interpret slip rate. Direct investigation of fault displacement using geologic piercing points has not been widely attempted. As a result three critical questions remain concerning the offset history of the Altyn Tagh fault: (1) What is the age of initiation of the Altyn Tagh fault? (2) How has the slip-rate on the Altyn Tagh fault varied over geologic time? (3) How does the magnitude of slip vary along the length of the Altyn Tagh fault? In order to answer these three questions it is essential to identify pre-slip and multiple syn-slip piercing points on each segment of the fault. These piercing points will allow the history of offset on the fault to be determined for each segment of the fault (western, central, and eastern) and thus demonstrate slip magnitude, slip rate, and variation in these parameters along the length of the fault. The proposed study will directly address the Tertiary slip history of the Altyn Tagh fault and the distribution of slip on the fault, by identifying multiple Eocene-Upper Pliocene sediment-source matches across the fault for ten Tertiary basins that are truncated by the Altyn Tagh fault. By identifying multiple piercing points of various ages from a single basin, the slip history for that segment of the fault ca be reconstructed throughout the Tertiary. Furthermore, by examining a series of basins along the length of the fault, along-strike changes in displacement can be documented. These results will provide a robust and unambiguous data set from which the timing and rate of faulting can be directly determined, and that will also allow ideas about systematic changes in slip magnitude along the fault to be tested. The PI's prior work on the Altyn Tagh fault (Ritts, 1998; Yue and Liou, 1999; Ritts and Biffi, 2000; Sobel et al., 2001; Yue et al., in review) has demonstrated their ability to identify and document piercing points of a variety of ages and a variety of types on the Altyn Tagh fault. The proposed research includes documentation of lithostratigraphy and chronostratigraphy, physical sedimentology and lithofacies, provenance, and paleocurrents of ten Tertiary sedimentary basins that are truncated by the Altyn Tagh fault. Two major field seasons will focus on the characterization of sedimentary basins as well as potential sediment source terranes on the opposite side of the fault. Fieldwork in the Tertiary basins will include description of measured sections, collection of paleocurrent and conglomerate composition data, and geological mapping. Fieldwork in potential sediment source areas will focus on lithologic description and verification of Chinese mapping, as well as collection of potential source units for analytical work. Analytical techniques will be applied to conglomerate clasts and potential source units in order to test sediment-source correlations that are proposed based on field data. These techniques will include petrographic examination, trace and major element geochemistry, and U-Pb and 40 Ar/ 39 Ar geochronology. Biostratigraphic analysis of mudstones will be used to confirm the ages of the Tertiary units. This study will constrain the geology of Tertiary basins in order to construct a series of time- displacement curves to characterize the history and along-strike variability of the Altyn Tagh fault. The resulting data will better define the role that the Altyn Tagh fault plays in accommodating Cenozoic convergence between India and Asia, and thereby distinguish the relative importance of mechanisms such as extrusion tectonics in continental deformation.