Interaction between shallow and deep structures in the southern Junggar fold-and-thrust belt, northern Tianshan, Northwest China, and the implications for earthquake hazards

Determining deformation sequences and interactions between shallow and deep structures within multiple detachment systems is crucial for fully comprehending geological processes in fold-and-thrust belts. However, kinematic models that include interactions among multiple sets of active detachments are rare. This study focuses on the western segment of the southern Junggar fold-and-thrust belt (SJT) in northern Tianshan, Northwest China, which comprises three subparallel fold-and-thrust belts with an east–west trend. We developed a new, comprehensive 4-D model of the SJT based on the interpretation of 2-D seismic reflection profiles, surface geological and geomorphological mapping, and forward modeling. Two active horizontal detachments were identified: the upper detachment of the southern Junggar fold-and-thrust belt (SJTU) and the lower detachment of the southern Junggar fold-and-thrust belt (SJTL) are localized near the base of the Eocene Anjihai and Cretaceous Tugulu formations, respectively. The shallow SJTU system involved the South Anjihai tectonic wedge and the “shallow” Huoerguos anticline, whereas the deep SJTL system contained the Halaand, Dunan, and “deep” Huoerguos anticlines. The geometry of growth strata in the forward modeling and reflection profiles revealed a complex active interaction and link between the shallow and deep structures. The total shortening accommodated by the SJTU and SJTL remained relatively constant, exhibiting a 24.5% decrease across the transfer zone. The sizes of these fault segments indicate that the SJT is capable of larger events than previously reported. Our findings demonstrate that out-of-sequence deformation among multiple detachments is a fundamental process in foreland fold-and-thrust belts with direct implications for assessing multifault earthquake hazards worldwide. This study not only clarifies how fault-related folds interact across varying depths but also highlights the utility of forward modeling in deciphering complex natural tectonic geometries.