Active thrust sheet deformation over multiple rupture cycles: A quantitative basis for relating terrace folds to fault slip rates

Many recent thrust fault earthquakes have involved coseismic surface faulting and surface folding strains. This multifaceted nature of active thrust sheet deformation can make attempts to quantify slip and slip rates from surface strains challenging and uncertain. We present new methods for integrating rec-ords of surface deformation, subsurface structure, and geochronology to investigate active deformation over multiple rupture cycles across the Southern Junggar Thrust in the southern Junggar Basin, NW China, from similar to 225 ka to present. Fluvial terraces preserve records of surface faulting as a prominent fault scarp where the Southern Junggar Thrust is surface-emergent. Terraces also exhibit progressive folding strains across fold scarps that are spatially coincident with subsurface fault-bend folds-constrained by seismic reflection data-along the Southern Junggar Thrust. We quantify the fault slip at depth implied by fold scarp relief along Holocene-aged terraces, and the results are corroborated by independent estimates of slip implied by fault scarp relief for the same terraces. Older terraces exhibit a distinct fanning of dips across fold scarps, suggesting active fault-bend folding kinematics involving a component of limb rotation. We developed quantitative relations for fault-bend folds between fault slip and fold dip using a mechanical, forward modeling approach. Using this novel method, we show how Southern Jung-gar Thrust slip rate has decelerated markedly, from similar to 4.1 mm/yr in the middle Quaternary to similar to 1.2 mm/yr throughout the Holocene. These results provide new insight into natural fault-bend folding kinematics and define innovative methods for elucidating accurate estimates of fault slip and slip rates from terrace folds in active thrust sheets.