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

帕米尔高原、塔吉克洼地和周边地区为解决大规模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，最明显的陆内俯冲的例子是位于塔吉克斯坦和吉尔吉斯斯坦境内的帕米尔和南天山之间的边界，在那里可能发生10-20毫米/年的局部陆内会聚。其次，帕米尔高原提供了青藏高原的缩影；尽管位于印度接近欧亚大陆的直接路径上，但变形速度似乎很慢，并以东西向伸展为主，通过正常的走滑断层作用。第三，塔吉克大萧条中许多看似独立的地壳区块似乎像近乎坚硬的区块一样，在一层弱盐层上移动。许多人认为，大陆地区所有尺度的变形都可以用这样的地块来描述。这一地区提供了一个特别好的块体运动试验区，具有简单的流变结构。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，南天山陆内俯冲的假说，包括估计俯冲是在单个错动上进行的，就像在大洋俯冲带上一样，还是在一个分布的区域上进行，就可以检验了。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。整个帕米尔高原将使用相同的观测方式来量化地表应变率及其空间安排，以测试变形发生缓慢是因为该地区被强大的岩石圈覆盖，还是因为施加在其边缘的单位长度的力与由于高、均衡补偿的地形平衡而产生的应力之间的平衡，从而使该地区承受较小的应力。最后，在塔吉克凹陷，单个地壳块体的GPS速度矢量观测将被用来检验许多小相互作用块体系统中的有限应变理论公式。这些类型的模型已被用于描述大陆变形的其他地区，特别是在青藏高原内，即使在很少的块体有直接速度观测的情况下，但以前还没有对多块体系统进行彻底的调查。为了解决这些问题，并扩大东亚区域形变的全球定位系统覆盖面，将在今后五年内对至少50个新的和现有的全球定位系统运动站点进行测量，并在塔吉克斯坦和吉尔吉斯斯坦进行至少3个连续的全球定位系统站点的测量。战役测量涉及使用GPS仪器的多个短期调查基准；连续测量涉及固定仪器连续记录数据。来自蒙大拿大学、科罗拉多大学、俄罗斯比什凯克科学院和塔吉克斯坦共和国科学院地震工程与地震研究所的科学家将对这些测量结果进行制作和解释。许多关于大陆如何构造变形的想法都是以青藏高原为试验对象的。然而，仅凭一个例子，就不可能弄清楚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。对这些地区地表形变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。正确地将大陆岩石圈纳入构造理论，对人类社会也具有更广泛的意义。大洋俯冲带是地球上最大的地震(如苏门答腊岛地震)的所在地。大陆俯冲带能产生同样大的事件吗？或者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评估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.