Mechanics and paleo geomorphy of the platform margin of Dengying formation in the Jiulongshan field, Sichuan Basin, China

The northwestern Sichuan region experienced the evolutionary process of a marine Craton basin in the Sinian-Middle Triassic and a continental basin in the Mesozoic-Cenozoic. Several regional tectonic activities cause the complicated stratigraphic distribution and structural deformations in deep layers. During key tectonic periods, the characteristic sedimentary and deformation structures were formed, including the platform margin of Dengying formation, the western palaeohigh at the end of Silurian, and the passive continental margin of late Paleozoic-middle Triassic. The Meso-Cenozoic intra-continental compressional tectonic processes since the late Triassic controlled the formation of complex thrusting structures surrounding and inside the basin. The northern Longmenshan fold-thrust belt has footwall in-situ thrust structures, controlled by two sets of detachments in the Lower Triassic and the Lower Cambrian, presenting a multi-level deformation structure with shallow folds, the middle thin-skin thrusts and the deeper basement-involved folds. From the perspective of structural geology, the Dengying formation of the Upper Sinian is mainly distributed in the eastern and northern areas of the northwest Sichuan basin where the Jiulongshan fold is the favorable exploration belts. Using the three-dimensional seismic reflection data, we recognize the structural characteristics of the platform margin of Dengying formation. Meanwhile, we apply new methods of two-dimensional and structural restoration based on mechanical constrains to gain insights into the development of the Jiulongshan anticline which forms the trap for the Jiulongshan field. The result of structural restoration indicates that, the formation of the Jiulongshan anticline is controlled by two-stage contractional thrusts. In the early days, there was no significant relief in Jiulongshan area, and the southwestern top of the Sinian Dengying formation was the paleo-high. The anticline was gradually formed in the Late Jurassic-the Early Cretaceous, presenting an approximately E-W strike structure. This structure was transformed by the N-E contractional stress to become an anticline in NE-SW direction.