3-D geomechanical restoration and paleomagnetic analysis of fault-related folds: An example from the Yanjinggou anticline, southern Sichuan Basin

We examine the development of the Yanjinggou anticline, a fault-propagation-fold in the southern Longmen Shan, through an integrated study of structural geometry, strain, and paleomagnetism. The 3-D structural and strain restoration models generated in our analysis reveal that the NE-trending Yanjinggou fold has a curved map trace that is convex to the southeast. The fold has three distinct regions characterized by different strain patterns: contraction in the core of the fold, extension in the outer arc, and a forelimb with distributed shear. To further understand the kinematics of the Yanjinggou anticline, we performed paleomagnetic analysis on 184 oriented samples collected across the structure. Anisotropy of magnetic susceptibility (AMS) measurements and stepwise thermal demagnetization were conducted. A strike test was applied to the high temperature component (HTC) in order to identify rotation around the arc. The result indicates that the Yanjinggou anticline is a progressive arc, with a minor initial curvature and a dominant secondary curvature related to vertical-axis rotation synchronous with thrusting. The primary curvature and initial development of the structure correlates with the growth of the southern Longmen Shan in Late Miocene. The secondary curvature correlates with displacement extending since Late Pleistocene toward the southeast into the central basin along the detachments that underlie the structure. Lateral gradients in displacement along this underlying detachment provide a mechanism for producing the vertical rotation of the anticline. AMS results and historical earthquake analysis imply that the fault-propagation fold, along with other NE trending structures in the southern Sichuan basin, are tectonically active and accommodate east-west crustal shortening in the basin. By integrating 3-D structural and strain restoration modeling with systematic AMS and paleomagnetic methods using statistical analysis, we closely constrain how the Yanjinggou anticline developed, and provide insights into the formation of fault-related folds with curved shapes in map view, which are common in other fold-and-thrust belts around the world.