Paleomagnetic constraints on Late Miocene kinematic adjustment in the western South Tianshan foreland

Summary The western South Tianshan foreland records late Cenozoic deformation associated with the India-Eurasia collision, yet the timing and nature of Miocene kinematic changes remain poorly constrained. Here we present new magnetostratigraphic, paleomagnetic, and anisotropy of magnetic susceptibility (AMS) data from a ~12–6 Ma succession in the western Keping fold-and-thrust belt (FTB). Paleomagnetic results from 39 site-mean directions reveal small-magnitude vertical-axis rotations since ~12 Ma, characterized by two distinct rotational phases: an earlier clockwise (CW) rotation (potentially reaching ~8-9° when evaluated relative to a younger counterclockwise (CCW) background) prior to ~10 Ma, followed by a persistent but minor CCW rotation (~4°) from ~10 to 6 Ma. Although the magnitude of rotation is limited, this pattern indicates a subtle change in rotational behavior. Importantly, the persistence of the CCW rotation throughout the younger interval suggests that the observed rotations were not fully acquired during deposition, but may have been modified by deformation younger than ~6 Ma. AMS fabrics show corresponding variations in magnetic anisotropy, indicating a shift from relatively organized tectonic strain to more distributed deformation. We interpret these results as reflecting progressive basinward propagation of thrusting in the South Tianshan foreland, with rotation-related deformation occurring during a relatively late stage. Overall, the dataset highlights the limited magnitude and young timing of rotational deformation within the Keping FTB.