Collaborative Research: GPS Study of the Kinematics of the India-Eurasia Convergence Zone across the Pamir and Adjacent Terrain

帕米尔、塔吉克坳陷及其周边地区为解决大尺度大陆地球动力学中的三个基本问题提供了非常好的野外实验室。首先，最明显的陆内俯冲例子位于塔吉克斯坦和吉尔吉斯斯坦的帕米尔和南天山之间的边界，在那里可能发生10-20 mm/年的局部陆内辐合。第二，帕米尔高原提供了一个缩小版的青藏高原；尽管处于印度接近欧亚大陆的直接路径上，但变形似乎是缓慢的，主要是通过正断层和走滑断层的东西延伸。第三，塔吉克坳陷的许多明显独立的地壳块体似乎在一层弱盐层上移动，几乎是刚性块体。许多人认为，这种块体最能描述大陆地区各种尺度的变形。该区域具有简单的流变结构，为块体运动提供了一个特别好的测试区域。GPS大地测量可以非常精确地观测地球表面的应变和位移。有了这些观测的适当网络，就可以检验南天山大陆内俯冲的假设，包括对俯冲是发生在单一位错上的估计，如在海洋俯冲带中，还是发生在一个分布区域上。在前一种情况下，GPS观测的反演提供了俯冲带几何形状的估计，包括走向、倾角和锁定深度。同样的观测方式将在整个帕米尔高原上使用，以量化地表应变率及其空间排列，以测试变形是否发生缓慢，是因为该地区处于强大的岩石圈之下，还是因为施加在其边缘的单位长度的力与由于高的、均衡补偿的地形平衡而使该地区承受很小压力的应力之间的平衡。最后，在塔吉克坳陷，单个地壳块体速度矢量的GPS观测将用于测试许多小相互作用块体系统中有限应变的理论公式。这些类型的模型已被用于描述大陆变形的其他区域，特别是在青藏高原内，即使只有很少的块体有直接的速度观测，但对多块体系统的彻底研究以前还没有做过。为了解决这些问题，并增加东亚区域变形的GPS覆盖范围，将在未来五年内对至少50个新的和现有的运动GPS站点进行测量，在塔吉克斯坦和吉尔吉斯斯坦至少有3个连续的GPS站点。运动测量包括使用GPS仪器对测量基准进行多次短期占领；连续测量包括固定仪器连续记录数据。来自蒙大拿大学、科罗拉多大学、俄罗斯科学院、比什凯克和塔吉克斯坦共和国科学院地震工程与地震学研究所的科学家将制作和解释这些测量结果。许多关于大陆如何在构造上变形的观点都是利用青藏高原作为试验而发展起来的。然而，仅凭一个例子，很难判断青藏高原的哪些特征是大陆变形的一般特征，哪些是西藏特有的特征。幸运的是，塔吉克斯坦和吉尔吉斯斯坦的帕米尔和天山地区也代表了活跃的大陆碰撞，甚至可能是真正的大陆俯冲。对这些地区地表变形的测量和物理模型的发展可以帮助我们更清楚地了解大陆是如何融入板块构造系统的。将大陆岩石圈正确地整合到构造理论中，对人类社会也具有更普遍的意义。海洋俯冲带是地球上最大地震的发生地（比如苏门答腊地震）。大陆俯冲带能产生同样大的事件吗？还是大陆以不同的方式对构造力做出反应？这些问题的答案对于估计和应对中亚地区的地震灾害具有重要意义。

The Pamir, Tajik Depression, and surrounding territory offer unusually good field laboratories for addressing three basic questions in large-scale continental geodynamics. First, the clearest example of intracontinental subduction lies at the boundary between the Pamir and the South Tien Shan in Tajikistan and Kyrgyzstan, where localized intracontinental convergence at 10-20 mm/yr may occur. Second, the Pamir provide a miniature version of the Tibetan Plateau; deformation seems to be slow and dominated by east-west extension through normal and strike-slip faulting despite being in the direct path that India approaches Eurasia. Third, many, apparently independent crustal blocks of the Tajik Depression seem to move as nearly rigid blocks over a layer of weak salt. Many think that deformation at all scales in continental regions is best described by such blocks. This region offers an especially good testing region of block movements with a simple rheological structure. GPS geodesy allows for very precise observations of strain and displacement at the surface of the earth. With an appropriate network of these observations, the hypothesis of intracontinental subduction in the South Tien Shan, including estimation of whether subduction is accommodated on a single dislocation, as in oceanic subduction zones, or over a distributed region can be tested. In the former case, inversions of GPS observations provide estimates of the subduction zone geometry, including strike, dip, and locking depth. The same style of observation will be used throughout the Pamir to quantify surface strain rates and their spatial arrangement to test whether deformation occurs slowly because the region is underlain by strong lithosphere or because the equilibrium between forces per unit length applied to its margins and stresses due to high, isostatically compensated topography balance to put the region under little stress. Finally, in the Tajik Depression, the GPS observations of velocity vectors for individual crustal blocks will be used to test theoretical formulations of finite strain in systems of many small interacting blocks. These types of models have been used to describe other regions of continental deformation, especially within the Tibetan Plateau, even when few blocks have direct velocity observations, but a thorough investigation of multiblock systems has not been previously made. To address these questions, and to increase GPS coverage of regional deformation in eastern Asia, the measurement of at least 50 new and existing campaign GPS sites, with at least 3 continuous GPS sites in Tajikistan and Kyrgzystan will be carried out over the next five years. Campaign measurements involve multiple short occupations of survey benchmarks with GPS instruments; continuous measurements involve fixed instruments recording data continuously. Scientists from the University of Montana, the University of Colorado, the Russian Academy of Sciences, Bishkek, and the Institute of Earthquake Engineering and Seismology of the Academia of Sciences of the Republic of Tajikistan will make and interpret these measurements.Many ideas about how continents deform tectonically have been developed using the Tibetan Plateau as a test. However, with only a single example, it is impossible to figure out which characteristics of the Plateau are general properties of continental deformation, and which are special features unique to Tibet. Fortunately, the Pamir and Tien Shan regions in Tajikistan and Kyrgystan also represent active continental collision, and maybe even true continental subduction. Measurements of surface deformation and development of physical models in those regions can help us to more clearly understand how the continents fit into the system of plate tectonics. Integrating continental lithosphere correctly into tectonics theory also has more general significance to human communities. Oceanic subduction zones host the biggest earthquakes on the planet (such as the Sumatra earthquake). Can continental subduction zones generate equally large events? Or do the continents respond to tectonic forces in different ways? The answers to these questions are important for estimating and responding to seismic hazards in Central Asia.