Deciphering interseismic strain accumulation and its termination on the central-eastern Altyn Tagh fault from high-resolution velocity fields

Abstract

• We provide a high-resolution geodetic strain rate model for the central-eastern ATF. • The strike-slip rate on the ATF decreases gradually between ∼90.5°E and ∼94°E. • The shear strain on the ATF terminates in a horsetail structure at ∼95°E. • Fault planes of the ATF are nearly vertical, with strike changes along the bends. • Uplift occurs around restraining bends, being higher at their inside corners. Investigating the present-day pattern of strain accumulation along the Altyn Tagh fault (ATF) in northwestern Tibet is critical for our broader understanding of deformation around large active strike-slip faults and the associated seismic hazards. Previous geodetic and geological studies show an eastward decrease of slip rate along the central-eastern ATF, but the spatial variation of the slip rates and the mechanism causing such variation are uncertain. Additionally, interseismic deformation around the restraining bends along the ATF and its pattern of termination towards its eastern end are also unclear. Here we derive surface velocities and strain rates around the central-eastern ATF system using Sentinel-1 and GNSS velocities. We estimate fault parameters including slip rate and locking depth for the ATF and other related active faults indicated by our strain rate maps using a Bayesian inversion approach. Our results show shear strain is mainly concentrated on the ATF between 86°E and 95°E. The strike-slip rate of the ATF remains constant at ∼8 mm/yr between 86°E and 90.5°E, before decreasing gradually to ∼4.5 mm/yr between 90.5°E and 94°E due to the crustal shortening across the Qaidam basin. Shear strain on the ATF is terminated in a horsetail structure at 95°E, where the strain is split into the motion along the Danghe Nanshan fault, Yema River – Daxue Shan fault and north Altyn Tagh fault. Our strain rate fields show fault planes are nearly vertical beneath the ATF, and there are obvious changes in the strike of the deep shearing part of the fault beneath the Akato Tagh bend and Aksay bend. The Akato Tagh and Pingding Shan bends feature higher peak strain rates and narrower width of the interseismic straining zone at the surface, compared to straight sections like the Xorkoli segment. We observe long-wavelength uplift signals in the East Kunlun Shan range, Altun Shan range, Qaidam basin, restraining bends along the ATF and the Qilian Shan – Nan Shan thrust belt. We consider uplift of the latter is controlled by the thrust motion of the active faults within it, as almost all shortening within it occurs across the fault zones. Uplift around restraining bends along the ATF exhibits a larger rate at their inside corners, which is evidence of potential vertical-axis rotation within the bends. Our results provide a noteworthy example of how the strain is accumulated and terminated on a large-scale intra-plate strike-slip fault.

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