Obstacle leads to local stress reorientation: Insights from the formation mechanism of the Maiduoshan superimposed fold in the Weiningbeishan fold-thrust belt, NW China

Abstract

The Maiduoshan superimposed fold, located southwest of the Weiningbeishan fold-thrust belt in the transitional zone of the Tibetan Plateau, Alxa Block, and Ordos Basin, has a lotus shape. There are more than five chevron-like hinge zones in its east, which indicates that this fold has undergone structural superposition. After a detailed structural analysis and the results of three sandbox models, we propose that the Maiduoshan superimposed fold formed due to the superposition of two asynchronous tectonic events. During the Middle–Late Triassic, E–W-trending close and tight folds formed under the N–S compression. During the Pliocene–Quaternary, the left-lateral simple shear of the Yantongshan Fault caused early E–W-trending close and tight folds to refold. In addition, the results of the sandbox models suggest obstacles to the northwest of Maiduoshan. We speculate that the concealed pluton, formed during the Middle Jurassic–early Early Cretaceous in the Jinchangzi–Errenshan area to the northwest of Maiduoshan, acted as an obstacle that hindered the drag folds (formed by the left-lateral simple shear of the Yantongshan Fault) from moving northwestward. Lastly, we propose a local stress reorientation mechanism to explain the formation of a strain field with obstacles. In such a mechanism, the different external stresses applied by obstacles at different locations lead to the reorientation of the local stresses at different locations.