Multi‐Phase Non‐Coaxial Orogenic Growth: Propagation of the Longmen Shan Thrust Belt on the Eastern Margin of the Tibetan Plateau

Abstract

AbstractReconstructing the deformation sequence and strain history of the Longmen Shan thrust belt (LSTB) has significant implications for the growths of the SW Qinling orogenic belt and the Tibetan Plateau. However, the Mesozoic–Cenozoic propagation of the LSTB remains poorly understood owing to weak constraints on strain variations. We conducted a comprehensive structural investigation and made measurements of anisotropy of magnetic susceptibility along the entire LSTB. Magnetic fabrics are represented by prevalent layer‐parallel shortening (LPS) fabrics, intersection lineation fabrics, and folding fabrics with magnetic lineations trending NE–SW, WSW–ENE, NW–SE, and N–S, corresponding to four compressional deformations. AMS results and structural analyses, revealed four phases of deformation (D1, D2, D3, and D4) that were characterized respectively by top‐to‐the‐SSE thrusting in the northern LSTB at >219–164 Ma, top‐to‐the‐SW thrusting in the central‐northern LSTB at >201–164 Ma, top‐to‐the‐SE thrusting of the LSTB at 125–5 Ma, and top‐to‐the‐east thrusting at 5–0 Ma in the LSTB. D1, D3, and D4 LPS fabrics exhibit a strain evolution that progressed from incipient deformation to pencil structure to cleavage fabrics, corresponding to moderately oblate, prolate, and oblate strain ellipsoids, respectively. The intersection lineation fabrics imply a hinterland‐ward progression from LPS fabrics to cleavage‐controlled fabrics and non‐coaxial superimposition of D2 on D1. Therefore, the early Mesozoic LSTB developed owing to the superimposition of D2 on D1 being non‐coaxial, likely suggesting southward growth of the SW Qinling orogenic belt, whereas the modern LSTB was formed by the superimposition of D4 on D3, implying non‐coaxial eastward growth of the Tibetan Plateau.