Kinematics and dynamics of the Pamir, Central Asia: Quantifying surface deformation and force balance in an intracontinental subduction zone

Abstract

Kinematic and dynamic models quantify deformation and force balance in the Pamir, a region undergoing the rare and poorly understood process of intracontinental subduction. We constrain a detailed kinematic model with 506 recent GPS velocities and Quaternary fault slip rates and show that the Pamir is organized like the Himalaya and Tibet, with regions of (1) localized strain rate ≥100e‐9/year along the Pamir Frontal Thrust System (the subduction interface), similar to the Himalaya, and (2) distributed north‐south compression and east‐west extension, similar to Tibet. Through standard thin viscous sheet methods we demonstrate that the lithospheric force balance in the Pamir is a combination of stresses caused by gravitational potential energy and India‐Eurasia convergence accommodated at a subduction interface, in this case the Pamir Frontal Thrust System. We find that strain rate and deviatoric stress patterns near the Pamir Frontal Thrust System are characteristic of a mature subduction zone, despite its initiation in continental lithosphere. Although the Pamir and Tibet are kinematically and dynamically similar, the Pamir is stiffer overall than Tibet, perhaps due to the presence of the highly arcuate, geometrically stiffened continental slab at depth.

Kinematics and Dynamics of the Pamir, Central Asia: Quantifying the Roles of Continental Subduction in Force Balance

“…We confirm the results of our previous kinematic model (Jay et al, ). North‐south compression and concomitant east‐west extension characterize strain rate patterns within most of the high topography of the Pamir, and velocities rotate to the west along the Pamir western boundary.…”

“…We calculate through a joint least squares inversion of GPS velocities and strain rates from Quaternary fault slip rates: where and are the modeled and observed average strain rate components for each grid area, respectively; and are the modeled and observed velocities, respectively; V ij , kl is the strain rate variance‐covariance operator; and C i , j is the velocity variance‐covariance operator (e.g., Haines et al, ; Haines & Holt, ; Holt, Chamot‐Rooke, et al, ; Holt, Shen‐Tu, et al, ). See Jay et al () for a more thorough explanation of kinematic model methods.…”

“…Recent seismic evidence (Kufner et al, ) points to Indian and Asian origins for Hindu Kush and Pamir slabs, respectively, although the tomographic and structural evidence for a Pamir slab is stronger than for a separate Hindu Kush slab. Geodetic velocity observations throughout the Pamir and surrounding regions indicate distributed north‐south compression, east‐west extension, and gravitational collapse to the west (Ischuk et al, ; Jay et al, ; Mohadjer et al, ; Zubovich et al, ). Pamir deformation, therefore, is analogous to the kinematics of the Himalayan arc and Tibetan Plateau despite opposite subduction polarities, a smaller length scale, and subduction initiation in continental versus oceanic lithosphere (Jay et al, ).…”

Along‐Strike and Downdip Segmentation of the Pamir Frontal Thrust and Its Association With the 1985Mw6.9 Wuqia Earthquake

“…At the eastern half of the PFT, approximately from the epicenter of the 1974 Markansu event to its eastern end (Figure ), a basal detachment is present at the base of the Cenozoic sequence (with a typical depth of 8–13 km) and separates the upper ramp from the lower ramp (Figure ; Sippl et al, ; Teshebaeva et al, ), causing significant downdip segmentation. Lateral variations of stratigraphic lithology and thickness, along with rotation of the compressional stress direction around the Pamir front (Jay et al, ; Pan et al, ), result in significant along‐strike segmentation of the upper ramp and likely the lower ramp. The segmentation characteristics and the seismic behavior of the Tuomuluoan segment, therefore, might be broadened to characterize the entire eastern half of the PFT.…”

Cumulative and Coseismic (During the 2016 Mw6.6 Aketao Earthquake) Deformation of the Dextral‐Slip Muji Fault, Northeastern Pamir Orogen

“…On account of such complicated mountain‐building processes and crustal boundary conditions, the recent deformation of the Pamir is characterized by a variety of types of faulting deformation as documented by neotectonic, geodetic, and seismologic studies: thrusting and shortening along its northern margin, sinistral‐transpressional slip along its western flank, possible dextral slip along its eastern flank, and significant east‐west extension within its interior (e.g., Arrowsmith & Strecker, ; Chapman et al, ; Chevalier et al, ; Cowgill, ; Ischuk et al, ; Jay et al, ; Li et al, ; Robinson et al, , , ; Schurr et al, ; Sippl et al, ; Sobel et al, , ; Thiede et al, ; Zubovich et al, ). This tectonic deformation is proposed to result from radial thrusting along the orogen margin (e.g., Cowgill, ; Pan et al, ; Strecker et al, ), gravitational collapse and westward extrusion of orogenic material (Jay et al, ; Kufner et al, ; Schurr et al, ; Thiede et al, ), oroclinal bending of the entire Pamir‐Western Himalayan region (Yin et al, ), clockwise rotation of the rigid Tarim basin (Schurr et al, ), and/o thermal and density effects related to a lithospheric tear fault (Sobel et al, ; Thiede et al, ). Despite these knowledge, details of the structures accommodating the tectonic deformation and the relationship between different types of faults remain highly debated (e.g., Chevalier et al, ; Robinson et al, , ) because the geometry and kinematics of major active faults have not yet been well constrained and geodetic measurements are not dense enough in the region (Figures a and b).…”