SEISMOTECTONIC DEFORMATION OF THE LITHOSPHERE IN THE PAMIR AND ADJACENT TERRITORIES

Abstract

The intracontinental collision zone including the Pamir and Tien Shan is of particular interest for investigating geodynamic processes in the crust and lithosphere. We analysed seismotectonic deformation (STD) in this area, using the method of calculations proposed by Yu.V. Riznichenko and S.L. Yunga. Seismicity distribution patterns across the area and in depth and average annual STD velocity rates (STD intensity, IƩ) were estimated from the data published in the ISC Catalogue (International Seismological Center, London), that includes the information on more than 56000 earthquakes recorded in the study area in 1902–2018. Areas of intense seismotectonic deformation are identified for different depth ranges. The estimated STD intensity IƩ has a maximum value of 1.76⋅10–7 year–1 in the southwestern part of the Pamir-Hindu Kush seismic zone (depth 100–300 km). In the junction zone of the West Tien-Shan and the Pamir (western Alai and Trans-Alai ridges, depth 0–50 km), it amounts to 1.25⋅10–7 year–1. Locations of seismically active zones migrate de pending on the depth. At a depth of 0–50 km, two active seismic zones are identified – the eastern Trans-Alai ridge and the central part of the Pamir–Hindu Kush. At depths below 50 km, the southern part of the Pamir–Hindu Kush seismic zone is seismically active. Earthquakes of the highest magnitudes occurred at depths of 100–300 km. We investigated the STD direction patterns using the focal mechanisms of 3276 earthquakes (1949–2018) and constructed the diagrams showing the main stress axes. For the majority of the analysed seismic events, the compression axis changed its direction in the 300–360° azimuth sector. Maps were constructed to show averaged source mechanisms and STD values calculated for the crust and upper mantle. From representative data sets, averaged mechanisms were reconstructed for the junction zone of the South Tien Shan, North Pamir and Tarim (depths of 0–50 km) and for the southwestern part of the Pamir–Hindu Kush seismic zone (depths more than 50 km). Using the STD maps, the directions of shortening and extension axes were determined, and different deformation settings were described. Based on the STD tensors calculated for depths of 0–50 km (crust), we estimated Lode – Nadai coefficients με, plane strain angles ω (according to S.L. Yunga) and the vertical component of the STD tensors, and determined their distribution patterns. Compression (0.6≤με≤1) takes place in the major part of the study area, while the Central and South Pamirs are subjected to extension with shear (–0.6<με<–0.2) and pure shear (–0.2≤με≤+0.2). The study area (except the Central and South Pamirs) is uplifting. A comparison shows that deformation models based on the STD technique and GPS data are fully consistent.