Partitioning of India-Eurasia convergence in the Pamir-Hindu Kush from GPS measurements

The study of continental collision zones is crucial for understanding continental tectonics, as it reveals the relationship between rheology and deformation localization. Unlike the relatively simple behavior of oceanic lithosphere, where plate motion is accommodated by narrow fault zones, continental lithosphere exhibits a range of vertical strength profiles due to lateral variations in temperature and composition. This leads to scale-dependent deformation mechanisms. At the scale of single faults (~10 km), elastic deformation is dominant. At the scale of mountain ranges (~100 km), plastic material underthrusting can explain topography and dynamics. At the scale of the Tibetan Plateau (~1000 km), a viscous sheet model is appropriate. The Pamir-Hindu Kush region, encompassing NW Pakistan, Afghanistan, Tajikistan, and Kyrgyzstan, provides an opportunity to investigate the importance of scale-dependent mechanisms, including viscous dynamics, at lengths less than 1000 km. Geodetic observations from this region offer valuable constraints on regional kinematics and slip rates on major faults.

Numerous studies have investigated the tectonics of the Pamir-Hindu Kush region. Previous work has focused on identifying major faults (e.g., Chaman, Darvaz-Karakul, Talas-Ferghana) and estimating their slip rates using geological and geophysical data. However, these studies often lacked comprehensive geodetic measurements to quantify the distributed deformation within the region. Studies of Tibet provide a valuable context. The Tibetan Plateau exhibits similar complexities in deformation, with a combination of fault slip and distributed strain. Models have been proposed to explain the deformation in Tibet, including those incorporating viscous flow.

A network of GPS sites was installed in the Pamir-Hindu Kush region (30°–44°N latitude, 60°–76°E longitude), supplemented by data from existing IGS sites. Daily site position estimates were computed using GAMIT software in a loosely constrained global reference frame. Site velocities were then estimated in ITRF05 and Eurasia-fixed reference frames using GLOBK. Major faults were compiled from various sources, and slip rates were estimated as an upper bound by considering the relative velocities between bracketing GPS sites, ignoring elastic strain and block rotations. In regions without major faults (Hindu Kush, central Pamir), velocities were calculated normal and parallel to the main topographic trend. East-west extension in the central Pamir was estimated using data from Gan et al. (2007) and the Khorog GPS site. Comparisons were made between geodetic slip rates and geological slip rates from the literature.

The study revealed distinct kinematic conditions: high relative rates (>10 mm/yr) across major faults (e.g., Chaman system); high rates where major faults are absent (e.g., central Pamir); low rates (<5 mm/yr) across some major faults (e.g., Talas-Ferghana, Herat); and low rates in deforming regions without clear major faults (e.g., Tajik Depression). Specifically, an upper bound of 18 ± 1 mm/yr of sinistral shear was found for the Gardiz-Mokur-Konar fault system. North-south shortening in the Hindu Kush was estimated to be 13.2 ± 1 mm/yr, with 9.9 ± 1 mm/yr of sinistral shear. East-west extension across the central Pamir was 8.8 ± 2 mm/yr. Shortening across the Darvaz-Karakul Fault was estimated at 11.4 ± 2 mm/yr. Shortening across the Tajik Depression was 6.2 ± 1 mm/yr. Shortening across the Main Alai Thrust was 11.8 ± 2 mm/yr. Little to no present-day slip was detected on the Herat Fault, and the Talas-Ferghana Fault showed significantly slower slip than previous estimates. The sum of slip rates on known large faults does not account for the total India-Eurasia convergence.

The Pamir-Hindu Kush region exhibits four distinct kinematic styles, similar to the Himalayan-Tibetan region. Both regions show major faults surrounding areas of high elevation and subdued relief, with shorter faults and lower slip rates within the plateaus. In both areas, distributed deformation contributes significantly to accommodating India-Eurasia convergence. The findings suggest that a significant portion of the convergence is absorbed through distributed deformation, rather than solely through major faults. The low slip rates on major boundary faults suggest that rapid lateral translation ('escape' or 'extrusion') of the interior is unlikely. The similarity between the Pamir and Tibet highlights the role of multiple deformation mechanisms in continental tectonics and justifies the use of a scale-dependent rheological model, including viscous mechanics, even for smaller regions like the Pamir.

This study demonstrates that India-Eurasia convergence in the Pamir-Hindu Kush region is accommodated by a combination of slip on major faults and distributed strain. The Pamir, despite its smaller size compared to Tibet, exhibits similar deformation styles, emphasizing the importance of considering both localized and distributed deformation in continental tectonics. Further research could focus on improving the geodetic network density to better constrain the deformation field, investigate the relationship between distributed strain and the spatial distribution of smaller faults, and refine rheological models to capture the complexities of continental deformation.

The sparse nature of the GPS network might underestimate the true slip rates on some faults and the extent of distributed deformation. The assumption of ignoring elastic strain and block rotations might introduce uncertainties in slip rate estimates, particularly for dip-slip faults. The study relies on existing geological data for fault locations and slip rates, which can have associated uncertainties. Further geodetic measurements are needed for a more comprehensive understanding of the region's tectonic behavior.