Timing and nature of intracontinental deformation at the northwestern margin of the Sichuan Basin, China: evidence from structural analysis and detrital zircon U-Pb ages

摘要

The theory of plate tectonics suggests that deformation occurs mainly along plate boundaries; however, compression can result in the formation of orogens and basins within intracontinental settings. During these two tectonic processes, the sedimentation and environmental changes occur in response to marginal and intracontinental deformation. Early Jurassic - Early Cretaceous deformation and basin formation along the Qinling orogenic belt and the northwestern Sichuan Basin in central-SW China are ideal for investigating a reactivated tectonic belt and basin formation. We studied the Lower Jurassic - Lower Cretaceous sedimentary sequences and structures along the northwestern margin of the Sichuan Basin, and obtained detrital zircon U-Pb ages for these rocks. The structures show that deformation migrated SE-wards and S-wards into the Sichuan Basin along the Longmen Shan, Micang Shan and Daba Shan tectonic belts during middle-late Mesozoic time. The Lower Jurassic oligomictic conglomerates have a smaller grain size and thicken towards the south, indicating protracted transport from a northern source. The conglomerates deposited near-source record post-orogenic south-vergent thrusting during the Late Triassic - Early Jurassic epochs. The Lower Cretaceous conglomerates and sandstones have multiple sources, which indicate that they were rapidly deposited near their source, synchronous with thrusting that occurred in response to coeval SE-wards and S-wards thrusting in the Longmen Shan and Daba Shan tectonic belts during the Late Jurassic - Early Cretaceous epochs. Detrital zircon grains from the Lower Jurassic - Lower Cretaceous sedimentary rocks yielded age peaks of 2600-2200, 1850-1600, 850-700, 540-400, 250-180 and 180-140 Ma. A comparison of these ages with those of surrounding exposed rocks indicates that the sediments in the northwestern Sichuan Basin were supplied from the Qinling orogenic belt, the northwestern Yangtze Block, the south margin of the North China Block and the Songpan-Garze Terrane. The youngest peaks of detrital zircon U-Pb ages at 207 and 159 Ma constrain the two stages of intracontinental shortening and highlight the link between intracontinental deformation and sedimentation.