Early Cenozoic paleo-stress field in the eastern margin of the Tibetan Plateau : Magnetic fabric records in the southwestern part of Sichuan Basin

The southwestern part of the Sichuan Basin is located on the eastern margin of the Tibetan Plateau, adjacent to the southern segment of the Longmenshan fold-thrust belt. The early Cenozoic red beds in this area recorded the uplift history and tectonic evolution of the eastern margin of the Tibetan Plateau. In this study, we applied magnetic fabric (i.e., anisotropy of magnetic susceptibility) analyses to the lower Paleogene stratigraphic section (including the Mingshan and Lushan formations) in Lushan area. A total of 548 samples were analyzed. Results show that magnetic foliations of the samples are parallel to the bedding, with magnetic lineations in a direction of NE-SW (39 degrees/219 degrees) and clustered K-3 (120.9 degrees +/- 1.3 degrees). Such a fabric is the type of initial deformation magnetic fabric, which was formed in the diagenetic stage of the strata and has not been overprinted by later folding and faulting. We therefore suggest that these magnetic fabric results recorded the early Cenozoic paleo-stress field of the study area, where deformation was dominated by shortening with a NW-SE maximum principal stress, perpendicular to the magnetic lineation. The late Cenozoic and present stress field of this area, revealed by structural, GPS and earthquake focal mechanism studies, is similar to that of the early Cenozoic, suggesting that the Longmenshan plateau margin may have been formed in the early Cenozoic.

Automated summary

The study applied magnetic fabric analysis to the lower Paleogene stratigraphic section in the Lushan area of the southwestern Sichuan Basin, revealing a paleo-stress field dominated by NW-SE shortening during the early Cenozoic deformation, perpendicular to the magnetic lineation. The late Cenozoic and present stress field of this area is similar to that of the early Cenozoic, suggesting the formation of the Longmenshan plateau margin in the early Cenozoic.