Interaction between shallow and deep structures in the Southern Junggar fold-and-thrust belt, northern Tianshan, China

Fold-and-thrust belts (FTBs) serve as crucial structural elements in regulating crustal shortening and deformation within continental interiors. They exhibit intricate geometric and kinematic characteristics, encompassing various fault-related folds and multiple sets of primary (active) detachment planes at varying depths. It is crucial to determine the sequence of deformation and the interaction between shallow and deep structures within the multiple detachment systems to comprehend geological processes fully in fold-and-thrust belts (FTBs). However, the kinematic model involving the interaction of multiple sets of active detachments, remains unexplored. This study focuses on the western segment of the Southern Junggar fold-and-thrust belt (SJT, also known as the Northern Tianshan FTB), comprising three nearly parallel thrust-fold belts with an east–west trend. We established a four-dimensional evolution model of the SJT based on the interpretation of two‐dimensional seismic reflection profiles and surface mapping, along with forward modeling of shallow and deep structures. The results revealed two sets of active detachments: upper (SJTU) and lower (SJTL) detachment. The SJTU contained the South Anjihai tectonic wedge and the “shallow” Huoerguos anticline while the SJTL contained the Halaand, Dunan, and “deep” Huoerguos anticlines. A comparison of the deformation patterns between the growth strata in the forward modeling and reflection profiles revealed a complex interaction and linkage between the shallow and deep structures. The tectonic landforms on the surface were a result of this interaction. The total amount of shortening remained relatively constant while the shortening accommodated by the SJTU and SJTL exhibited a 24.5% decrease (from west to east) across the transfer zone. Our study contributes to the quantification of shortening transfer between the shallow and deep structures in FTBs and advances the current literature on the mechanisms of crustal shortening. Finally, based on shallow and deep structural interactions and cascading rupture, a multi-scale seismic rupture model for the SJT was proposed, and maximum magnitude was estimated. The cascading rupture of multiple faults raises the upper limit earthquake magnitude, and leads to a greater variety of energy accumulation mechanisms as more faults interact, resulting in the occurrence of strong earthquakes. This also necessitates a reassessment of the seismic hazards associated with such complex foreland thrust belts.