This work first presents field structural analysis, anisotropy of magnetic susceptibility (AMS) measurements, and kinematic and microstructural studies on the Neoproterozoic Pengguan complex located in the middle segment of the Longmenshan thrust belt (LMTB), NE Tibet. These investigations indicate that the Pengguan complex is a heterogeneous unit with a ductilely deformed NW domain and an undeformed SE domain, rather than a single homogeneous body as previously thought. The NW part of the Pengguan complex is constrained by top‐to‐the‐NW shearing along its NW boundary and top‐to‐the‐SE shearing along its SE boundary, where it imbricates and overrides the SE domain. Two orogen‐perpendicular gravity models not only support the imbricated shape of the Pengguan complex but also reveal an imbrication of high‐density material hidden below the Paleozoic rocks on the west of the LMTB. Regionally, this suggests a basement‐slice‐imbricated structure that developed along the margin of the Yangtze Block, as shown by the regional gravity anomaly map, together with the published nearby seismic profile and the distribution of orogen‐parallel Neoproterozoic complexes. Integrating the previously published ages of the NW normal faulting and of the SE directed thrusting, the locally fast exhumation rate, and the lithological characteristics of the sediments in the LMTB front, we interpret the basement‐slice‐imbricated structure as the result of southeastward thrusting of the basement slices during the Late Jurassic‐Early Cretaceous. This architecture makes a significant contribution to the crustal thickening of the LMTB during the Mesozoic, and therefore, the Cenozoic thickening of the Longmenshan belt might be less important than often suggested.