Early Eocene Compressional Deformation in the Northeastern Tibetan Plateau Recorded by Magnetic Fabrics of the Longzhong Basin

Abstract Intracontinental deformation records how early Eocene India–Asia collisional stress propagated into continental interiors. How rapidly compression reached the far field, and whether coeval Paleo‐Pacific slab dynamics contributed, remain debated. The Longzhong Basin, ∼1,900 km north of the collisional zone, provides a key test. Yet its early tectonic regime is contested because direct strain records are scarce. Here, we applied anisotropy of magnetic susceptibility, corrected using vertical‐axis rotation, to Cretaceous–early Eocene strata of the Xining and Linxia sub‐basins. Cretaceous fabrics define broadly E–W K1 orientations with ambiguous kinematic significance, whereas early Eocene fabrics consistently record layer‐parallel shortening. This contrast indicates that collisional compression reached the northeastern Tibetan Plateau within a few million years of collision onset. By the early Eocene, India‐Asia convergence, rather than Paleo‐Pacific rollback, dominated deformation in the Longzhong region, implying efficient stress transfer through mechanically coupled Tibetan terranes.