Geometry and kinematics of superimposed detachment folds: A case study of the Longdongping Anticline in the southeastern Sichuan Basin, China

Abstract

Quantitative analysis of the kinematics of superimposed detachment folds remains challenging in fold-thrust belts with multiple detachments. This study investigates the geometry and kinematics of a superimposed detachment fold—Longdongping anticline, Southeastern Sichuan Basin, China. 3D seismic reflection and borehole data image a detachment fold comprising an asymmetric Cambrian evaporite-cored box fold superimposed by fault-propagation fold geometry, overlain by a gentle Ordovician-Jurassic detachment anticline. Both structural culminations show no significant migration. Balanced cross sections quantify lithology-controlled differential shortening across structural layers, documenting negligible values in the basal layer, 0.66 km in the lower layer, a peak of 1.23 km in the middle layer, and 0.83 km in the upper layer. Thermochronology, restoration, and kinematic modeling reveal that the Longdongping Anticline experienced progressive multi-stage deformation since the Late Cretaceous, involving early detachment folding and subsequent fault-controlled modification under continuous regional compression. 2D kinematic modeling which can be explained by an open-system model demonstrates a composite deformation mechanism, where the basal Cambrian evaporites deform mainly by pure shear and the overlying competent strata by flexure, with fault slip accommodating strain differences. This deformation reflects vertical deformation decoupling and shortening partitioned among multiple detachment horizons. Comparable superimposed deformation patterns in the eastern Sichuan and Dabashan fold–thrust belts suggest that their deformation mechanisms are analogous to those governing the Longdongping Anticline. The results provide new insights into the kinematic evolution of multilayer detachment folds and fault superimposition in foreland fold–thrust systems.