Strain Partitioning and Strike‐Slip Faulting in the Nanjieshan, North Tibetan Foreland: Progressive Widening of the Altyn Tagh Fault System With Implications for Regional Earthquake Hazards

Abstract Large continental strike‐slip fault systems typically comprise a principal displacement zone and subordinate splay faults, exhibiting complex geometries, connectivity, and kinematics. Understanding the three‐dimensional arrangement of major and minor faults is important for determining strain distribution, landform development, and potential earthquake behavior. This study examines the Quaternary deformation, fault kinematics, and late Holocene ruptures along the Nanjieshan Fault system (NJSF), a major branch of the Altyn Tagh Fault (ATF) system in the North Tibetan Foreland. The E‐W‐trending NJSF comprises a principal strike‐slip fault flanked by two thrust faults, forming a regional, sinistral positive flower structure. Cosmogenic exposure dating, field data and remote sensing observations indicate that the strike‐slip rate of the main NJSF is ∼0.5 mm/a since ∼45 ka, consistent with the low slip rates observed on other faults northwest of the ATF. Paleoseismological trenches reveal three surface‐rupturing events on the NJSF in the last 2,500 years that may coincide temporally with the last two major earthquakes documented on the principal ATF, suggesting quasi‐simultaneous rupture behavior. Coulomb stress change modeling indicates that a future ATF earthquake is unlikely to trigger failure along the NJSF. Conversely, rupture of the NJSF could induce stress loading on the ATF, potentially bringing it to failure. A synchronous rupture on the ATF and its major branch faults could be facilitated by mechanically weakened fault rocks and supra‐hydrostatic fluid pressures within the deep southward‐dipping branch faults of the North Tibetan foreland. We conclude that improved prediction of future slip behavior along major strike‐slip fault systems requires detailed geochronological analysis of paleo‐earthquake events along subordinate branching faults.