Collaborative Research: Extrusion and Rotation during Intracontinental Deformation: The Role of the Kunlun Fault in the Indo-Asia Collision

尽管对大陆岩石圈对碰撞造山作用的力学和热响应的理解最近取得了进展，但仍然存在重要的争议。其中一个中心是大型走滑断层在陆内变形中的作用，以及这些构造是否1)响应大陆汇聚而控制准刚性块体的侧向‘逃逸’(例如，Tapponnier等人，1982)，或2)反映在普遍变形和剪切的地壳中应变的被动局部化(例如，England和Molnar，1990)。这两个模型对走滑断层的位移变化、断层位移与周围地壳块体变形的关系以及断层末端滑动的容纳性质做出了截然不同的预测。在西藏东部，关于活动变形性质的持续争论在很大程度上反映了这些预测的严格地质检验的有限数量。昆仑断裂是青藏高原中东部的一级构造特征，它提供了在走滑断裂在藏东活动变形中所起作用的相互竞争的假说中进行检验的关键机会。尽管全新世的滑动速率似乎在断裂中央的约11 mm/年是一致的(Van der Woerd等人，2000)，但一些观测表明，沿东昆仑断裂的显著左旋剪切没有到达青藏高原的边缘：1)在~102 E以东，无法分辨出断裂的活动轨迹(如Tapponnier和Molnar，1977)；2)野外观测(Kirby)证实，在该地区以东不存在与昆仑断裂有关的斑块；3)大地测量表明，目前几乎没有可分辨的左旋剪切穿过高原东缘(Chen等，2000)。确定沿昆仑断裂最东端的左旋剪切对了解其在西藏东部变形中的运动学和动力学作用，以及更广泛地说走滑断裂在陆内变形中的作用是至关重要的。Pi提出了检验几个关于陆内走滑断裂表面终止时位移转移和/或调节机制的假说：CE假说1：位移转移到运动学联系的走滑断裂：a.将位移传递到高原边缘或球体之外。将位移传递到高原边缘的缩短构造。CE假设2：位移被高原内的分布缩短吸收，导致地壳增厚。CE假设3：位移代表断层在扩散的顺时针区域剪切作用下的被动旋转。检验这些假设将集中在以下任务：CE确定昆仑断裂最东段的晚更新世-全新世滑动速率，特别注意沿走向的潜在变化。建立几何学、运动学、以及候选调节构造(高原内和其边缘)上的位移速率。这项研究评估了安耶马群山(高原内缩短的主要候选者)差异岩石隆起和河流切割的规模和分布。这项研究承诺带来详细的年代学观点，以揭示大陆内大型走滑断裂终点处应变的性质。PI将记录此类结构末端附近存在或不存在的位移梯度。这项研究将确定断层位移与区域变形模式的关系，并将确定位移转移到其他构造的一些机制。最后，它将确定断层位移与边界块体变形的联系程度。合并后的结果将对大陆变形过程中挤压与旋转的问题产生关键的新见解。

Despite recent advances in understanding of the mechanical and thermal response ofcontinental lithosphere to collisional orogenesis, important controversies remain. One of these centers on the role of large strike-slip faults during intracontinental deformation, and whether these structures 1) control the lateral 'escape' of quasi-rigid blocks in response to continental convergence (e.g., Tapponnier et al., 1982), or 2) reflect the passive localization of strain in a pervasively deforming and shearing crust (e.g., England and Molnar, 1990). The models make very different predictions regarding the variation of displacement along strike-slip faults, the relationship of fault displacement to deformation of the surrounding crustal blocks, and the nature of accommodation of slip at the terminations of the faults. In eastern Tibet, continuing debate over the nature of active deformation reflects, to a large degree, the limited number of rigorous geologic tests of these predictions.The Kunlun fault is a first-order structural feature in the central and eastern Tibetan Plateau, where it presents a key opportunity to test among competing hypotheses for the role of strike-slip faults in the active deformation of eastern Tibet. Although Holocene slip rates appear to be uniform at ~11mm/yr along the central portion of the fault (Van der Woerd et al., 2000), several observations suggest that significant left-lateral shear along the eastern Kunlun fault does not reach the margin of the Tibetan Plateau: 1) the active trace of the fault on remote sensing (e.g., Tapponnier and Molnar, 1977) cannot be distinguished east of ~102 E; 2) field observations (Kirby) confirm that scarps associated with the Kunlun fault are not present east of this region; and 3) geodetic surveys indicate that, at present, little resolvable left-lateral shear passes through the eastern margin of the plateau (Chen et al., 2000). Determining what happens to left-lateral shear along the easternmost portion of the Kunlun fault is critical if we are to understand its kinematic and dynamic role in deformation of eastern Tibet and more generally the role of strike-slip faults during intracontinental deformation.The PI's propose to test several hypotheses regarding the mechanisms of transfer and/oraccommodation of displacement at the apparent termination of an intracontinental strike-slip fault: ce Hypothesis 1: Displacement is transferred to kinematically linked, strike-slip faults that:a. transmit displacement across and beyond the plateau margin, orb. transmit displacement to shortening structures at the plateau margin.ce Hypothesis 2: Displacement is absorbed by distributed shortening within the plateau resulting in crustal thickening.ce Hypothesis 3: Displacement represents passive rotation of faults in response to a diffuse, clockwise regional shear.Testing these hypotheses will focus on the following tasks:ce Determining Late Pleistocene-Holocene slip rates along the easternmost segment of the Kunlun fault, with special attention to potential variations along strike.ce Establishing the geometry, kinematics, and rates of displacement on candidate accommodation structures (both within the plateau and at its margin).ce Assessing the magnitude and distribution of differential rock uplift and river incision in the Anyemaqen Shan (the prime candidate for shortening within the plateau)This study promises to bring a detailed chronologic perspective to bear on the nature ofaccommodation of strain at the terminations of large, intracontinental strike-slip faults. The PI's will document the presence or absence of displacement gradients present near the ends of such structures. The study will define the relationship of fault displacement to regional deformation patterns and will determine some of the mechanisms by which displacement is transferred to other structures. Finally, it will determine to what degree fault displacements are linked to deformation of the bounding blocks. The combined results will yield critical new insights into the problem of extrusion versus rotation during continental deformation.