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.
In continental collisions, tectonic deformation of the Earth’s surface is accommodated by a wide range of processes including earthquakes on faults, mountain building, and movement of crustal materials out of a collision zone. The attached map shows the Pamir and Hindu Kush regions of Central Asia, where India collides with Asia. Vectors on the attached map, created by making precise position observations over five years, illustrate the velocity of the surface due to these tectonic processes. Where vectors vary in length or direction in space, they show variations in deformation mechanism and constrain the rates of that deformation. For example, the decreasing vector length from central Pakistan (NCEG) across the Hindu Kush to southern Tajikistan (ISHA + MANM) shows that there is active mountain building in progress in the Hindu Kush, and that the Tirich Mir and Reshun Faults have some earthquake risk. In contrast, the similarity of velocities within the Pamir (from ABLA to UBLA) shows little deformation or earthquake risk in eastern Tajikistan. The velocity field as a whole indicates that deformation in the Pamir is mostly localized around the edges of the region of high topography, unlike in the Tibetan Plateau to the east. This result further indicates that tectonic forces are accommodated very differently from one continental region to another, challenging the standard model of plate tectonics. These results are used to advance our basic physical understanding of the Earth’s lithosphere and plate tectonic processes. They are also used to systematically assess the hazard related to large earthquakes throughout Central Asia, and to educate residents of the region about their exposure to these hazards. Quantifying and effectively communicating risk allows for better planning and preparation, because limited resources can be invested in the most dangerous areas with the consent of stakeholders.
