Topographic loading-dominated kinematics of the fold-and-thrust belts: Sandbox modeling of the Longmen Shan thrust belt in eastern Tibetan Plateau

Abstract

The influence of topographic loading on the thrust kinematics of fold-and-thrust belts has long been poorly understood. The current Longmen Shan thrust belt (LSTB) in the eastern Tibetan Plateau has experienced progressive deformation, which might be influenced by preexisting topographic loading. Sandbox modeling was conducted herein to investigate the deformation of the LSTB. A reference model without topographic slope and five models with slopes of 6°, 14°, 17°, 19°, and 22°, each with two décollements, were evaluated using a sandbox instrument and particle image velocimetry. The critical slope angle for the experimental materials was determined to be 15°–17°. Subcritical wedges facilitated the formation of imbricate forethrusts and triangle zones with large-displacement backthrusts at the shallow level and active roof duplexes at the deep level. Furthermore, critical-supercritical wedges promoted the nucleation of imbricate forethrusts and rudimentary triangle zones with small-displacement backthrusts at the shallow level and passive roof duplexes at the deep level. The different kinematic assemblages were dominated by the localization of back-thrusting as topographic loading varied. The preferred deep backthrusts were likely associated with a brittle-ductile shear stress ratio exceeding 13.5 and with rotation of the stress σ1 axis towards hinterland. However, the shallow backthrusts tended to vanish due to increased ductile layer strength as initial backthrust tips showed increased horizontal velocity differences. Therefore, this study suggests that the Mesozoic topography probably created critical-supercritical to subcritical wedges from northern to southern segments, dominating the along-strike differential evolution of triangle zones and duplexes in the LSTB during the Cenozoic.