Structural analysis of the foreland basin of the southern Longmenshan tectonic belt: New insights from magnetic fabrics

Multi-scale intracontinental deformations were developed in response to the tectonic evolution of the southern Longmenshan tectonic belt (LMS) on the eastern margin of the Tibetan Plateau. Detailed studies, including field structural analysis and anisotropy of magnetic susceptibility (AMS), were conducted on sedimentary rocks in the foreland basin of the southern Longmenshan tectonic belt (FBSL) in order to better understand the tectonic evolution of the southern LMS. Field observations show that FBSL deformations are mainly characterized by broad and gentle folds in a NE-SW direction and shallow thrust faults. At least two sedimentary discontinuities and two-stage deformations occurred after the deposition of Cretaceous strata. Samples of magnetic fabrics from the Qionglai-Changshiba profile were characterized by the triaxial magnetic susceptibility ellipsoids common in sedimentary rocks. The magnetic fabrics of the profile represent a weak deformation that was associated with layer-parallel shortening before folding and mainly reflect the Mesozoic-Cenozoic NW-SE convergence. There are also the following atypical magnetic fabrics: magnetic foliation that is oblique to the bedding and that was associated with layer-parallel simple shearing during folding; and magnetic lineation that is to varying degrees oblique to the strike of bedding and represents a superposition of structures. Based on the correlation results of magnetic fabrics for samples of different ages, a tectonic superposition since the Late Indosinian Orogeny was found in FBSL. By combining field structural analysis and magnetic fabrics, it was suggested herein that the FBSL is a propagation product of Mesozoic-Cenozoic deformations of southern LMS to the southeast, which therefore experienced a composite superposition of deformations. This finding provides the basis for a comprehensive understanding of the southern LMS and may shed light on the uplift of the eastern margin of the Tibetan Plateau and the tectonic response of its eastern boundary.