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

阿尔金塔格断层系统，描绘了青藏高原的北部边缘，是印度和欧亚大陆碰撞的最大和最引人注目的特征之一，但也是最神秘的特征之一。尽管人们对阿尔金-塔格断裂带的构造地质和滑动历史的了解普遍较差，但它的构造意义一直是人们猜测的主题，而且该断裂带在喜马拉雅-西藏造山带的模型中占有重要地位。最近，人们试图更好地了解Altyn Tagh断层，主要是使用间接方法来解释断层开始的年龄和位移的大小，或者使用大地测量技术来解释滑动速率。利用地质穿刺点直接研究断层位移还没有得到广泛的尝试。因此，Altyn Tagh断层的偏移史仍然存在三个关键问题：(1)Altyn Tagh断层的形成年龄是什么时候？(2)阿尔金断裂带的滑动速率在不同地质时期是如何变化的？(3)沿Altyn Tagh断层的长度，滑动震级如何变化？为了回答这三个问题，必须在断层的每一段上确定预滑和多个顺滑刺穿点。这些穿刺点可以确定断层上每段断层（西部、中部和东部）的断层偏移历史，从而显示断层的滑动幅度、滑动速率以及这些参数沿断层长度的变化。本研究通过对阿尔金断裂带截断的10个第三系盆地进行始新世-上新世沉积源匹配，直接研究阿尔金断裂带第三系滑动史和断层上的滑动分布。通过在一个盆地中识别多个不同年龄的刺穿点，可以重建该断裂段在整个第三纪的滑动历史。此外，通过检查沿断层长度的一系列盆地，可以记录位移沿走向的变化。这些结果将提供一个可靠的、明确的数据集，从中可以直接确定断层的时间和速率，这也将允许对沿断层滑动幅度的系统变化进行测试。PI先前对Altyn Tagh断层的工作（Ritts, 1998； Yue和Liou, 1999； Ritts和Biffi, 2000； Sobel等人，2001；Yue等人，回顾）已经证明他们有能力识别和记录Altyn Tagh断层上各种年龄和各种类型的刺穿点。研究内容包括：阿尔金断裂带截断的10个第三系沉积盆地的岩石地层学和年代地层学、物性沉积学和岩相、物源和古流的文献记录。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源地体。在第三纪盆地的野外工作将包括对测量剖面的描述、古海流和砾岩成分数据的收集以及地质制图。在潜在沉积物源区的实地工作将侧重于岩性描述和中国测绘的验证，以及收集潜在的沉积物源区单位进行分析工作。分析技术将应用于砾岩碎屑和潜在的来源单位，以便测试根据现场数据提出的沉积物-来源相关性。这些技术将包括岩石学检查、微量元素和主要元素地球化学、U-Pb和40 Ar/ 39 Ar地质年代学。泥岩的生物地层分析将用于确定第三纪单元的年龄。本研究将约束第三系盆地的地质条件，建立一系列的时间-位移曲线，以表征阿尔金断裂的历史和沿向变异性。所得数据将更好地界定阿尔金塔格断裂在容纳印度和亚洲之间新生代辐合中的作用，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.